blkmap: Handle read-only slices in write path

Some blkmap slices (like blkmap_crypt) don't support writes and have
their write function set to NULL. The blkmap_blk_write_slice() function
calls the write function without checking if it's NULL, causing a crash
when attempting to write to such slices.

Add a NULL check before calling the write function. When the slice
doesn't support writes, return 0 to indicate no blocks were written.

Co-developed-by: Claude <noreply@anthropic.com>
Signed-off-by: Simon Glass <simon.glass@canonical.com>

drivers/block/blkmap.c

@@ -281,6 +281,9 @@ static ulong blkmap_blk_write_slice(struct blkmap *bm, struct blkmap_slice *bms,
 {
 	lbaint_t nr, cnt;
 
+	if (!bms->write)
+		return 0;
+
 	nr = blknr - bms->blknr;
 	cnt = (blkcnt < bms->blkcnt) ? blkcnt : bms->blkcnt;
 	return bms->write(bm, bms, nr, cnt, buffer);