extmod/vfs_blockdev: Add extended read/write methods.

This commit adds helper functions to call readblocks/writeblocks with a
fourth argument, the byte offset within a block.

Although the mp_vfs_blockdev_t struct has grown here by 2 machine words, in
all current uses of this struct within this repository it still fits within
the same number of GC blocks.

extmod/vfs.h

@@ -59,8 +59,8 @@ typedef struct _mp_vfs_proto_t {
 typedef struct _mp_vfs_blockdev_t {
     uint16_t flags;
     size_t block_size;
-    mp_obj_t readblocks[4];
-    mp_obj_t writeblocks[4];
+    mp_obj_t readblocks[5];
+    mp_obj_t writeblocks[5];
     // new protocol uses just ioctl, old uses sync (optional) and count
     union {
         mp_obj_t ioctl[4];
@@ -80,7 +80,9 @@ typedef struct _mp_vfs_mount_t {
 
 void mp_vfs_blockdev_init(mp_vfs_blockdev_t *self, mp_obj_t bdev);
 int mp_vfs_blockdev_read(mp_vfs_blockdev_t *self, size_t block_num, size_t num_blocks, uint8_t *buf);
+int mp_vfs_blockdev_read_ext(mp_vfs_blockdev_t *self, size_t block_num, size_t block_off, size_t len, uint8_t *buf);
 int mp_vfs_blockdev_write(mp_vfs_blockdev_t *self, size_t block_num, size_t num_blocks, const uint8_t *buf);
+int mp_vfs_blockdev_write_ext(mp_vfs_blockdev_t *self, size_t block_num, size_t block_off, size_t len, const uint8_t *buf);
 mp_obj_t mp_vfs_blockdev_ioctl(mp_vfs_blockdev_t *self, uintptr_t cmd, uintptr_t arg);
 
 mp_vfs_mount_t *mp_vfs_lookup_path(const char *path, const char **path_out);

extmod/vfs_blockdev.c

@@ -60,6 +60,19 @@ int mp_vfs_blockdev_read(mp_vfs_blockdev_t *self, size_t block_num, size_t num_b
     }
 }
 
+int mp_vfs_blockdev_read_ext(mp_vfs_blockdev_t *self, size_t block_num, size_t block_off, size_t len, uint8_t *buf) {
+    mp_obj_array_t ar = {{&mp_type_bytearray}, BYTEARRAY_TYPECODE, 0, len, buf};
+    self->readblocks[2] = MP_OBJ_NEW_SMALL_INT(block_num);
+    self->readblocks[3] = MP_OBJ_FROM_PTR(&ar);
+    self->readblocks[4] = MP_OBJ_NEW_SMALL_INT(block_off);
+    mp_obj_t ret = mp_call_method_n_kw(3, 0, self->readblocks);
+    if (ret == mp_const_none) {
+        return 0;
+    } else {
+        return MP_OBJ_SMALL_INT_VALUE(ret);
+    }
+}
+
 int mp_vfs_blockdev_write(mp_vfs_blockdev_t *self, size_t block_num, size_t num_blocks, const uint8_t *buf) {
     if (self->writeblocks[0] == MP_OBJ_NULL) {
         // read-only block device
@@ -79,6 +92,24 @@ int mp_vfs_blockdev_write(mp_vfs_blockdev_t *self, size_t block_num, size_t num_
     }
 }
 
+int mp_vfs_blockdev_write_ext(mp_vfs_blockdev_t *self, size_t block_num, size_t block_off, size_t len, const uint8_t *buf) {
+    if (self->writeblocks[0] == MP_OBJ_NULL) {
+        // read-only block device
+        return -MP_EROFS;
+    }
+
+    mp_obj_array_t ar = {{&mp_type_bytearray}, BYTEARRAY_TYPECODE, 0, len, (void*)buf};
+    self->writeblocks[2] = MP_OBJ_NEW_SMALL_INT(block_num);
+    self->writeblocks[3] = MP_OBJ_FROM_PTR(&ar);
+    self->writeblocks[4] = MP_OBJ_NEW_SMALL_INT(block_off);
+    mp_obj_t ret = mp_call_method_n_kw(3, 0, self->writeblocks);
+    if (ret == mp_const_none) {
+        return 0;
+    } else {
+        return MP_OBJ_SMALL_INT_VALUE(ret);
+    }
+}
+
 mp_obj_t mp_vfs_blockdev_ioctl(mp_vfs_blockdev_t *self, uintptr_t cmd, uintptr_t arg) {
     if (self->flags & MP_BLOCKDEV_FLAG_HAVE_IOCTL) {
         // New protocol with ioctl