linux: Add BDEVNAME_SIZE to blkdev.h

Add BDEVNAME_SIZE definition to blkdev.h, matching the Linux kernel
header location for this constant.

Co-developed-by: Claude Opus 4.5 <noreply@anthropic.com>
Signed-off-by: Simon Glass <simon.glass@canonical.com>

include/linux/blkdev.h

@@ -12,6 +12,9 @@
 struct block_device;
 struct gendisk;
 
+/* Largest string for a blockdev identifier */
+#define BDEVNAME_SIZE	32
+
 /* Block size helpers */
 #define bdev_logical_block_size(bdev)	512