btrfs: add nr_global_roots to the super block definition

We already have this defined in btrfs-progs, add it to the kernel to
make it easier to sync these files into btrfs-progs.

Signed-off-by: Josef Bacik <[e-mail]>
Signed-off-by: David Sterba <[e-mail]>

fs/btrfs/accessors.h

···
 BTRFS_SETGET_STACK_FUNCS(super_magic, struct btrfs_super_block, magic, 64);
 BTRFS_SETGET_STACK_FUNCS(super_uuid_tree_generation, struct btrfs_super_block,
 			 uuid_tree_generation, 64);
+BTRFS_SETGET_STACK_FUNCS(super_nr_global_roots, struct btrfs_super_block,
+			 nr_global_roots, 64);
 
 /* struct btrfs_file_extent_item */
 BTRFS_SETGET_STACK_FUNCS(stack_file_extent_type, struct btrfs_file_extent_item,

include/uapi/linux/btrfs_tree.h

···
 	/* The UUID written into btree blocks */
 	__u8 metadata_uuid[BTRFS_FSID_SIZE];
 
+	__u64 nr_global_roots;
+
 	/* Future expansion */
-	__u8 reserved8[8];
 	__le64 reserved[27];
 	__u8 sys_chunk_array[BTRFS_SYSTEM_CHUNK_ARRAY_SIZE];
 	struct btrfs_root_backup super_roots[BTRFS_NUM_BACKUP_ROOTS];