FAT: Add new ioctl VFAT_IOCTL_GET_VOLUME_ID for reading the volume ID.

Signed-off-by: Brian Swetland <swetland@google.com>
diff --git a/fs/fat/fat.h b/fs/fat/fat.h
index 8276cc2..74fe5d3 100644
--- a/fs/fat/fat.h
+++ b/fs/fat/fat.h
@@ -78,6 +78,7 @@
 	const void *dir_ops;		     /* Opaque; default directory operations */
 	int dir_per_block;	     /* dir entries per block */
 	int dir_per_block_bits;	     /* log2(dir_per_block) */
+	unsigned long vol_id;        /* volume ID */
 
 	int fatent_shift;
 	struct fatent_operations *fatent_ops;