btrfs: open code trivial helper btrfs_header_fsid

The helper btrfs_header_fsid follows naming convention of other struct
accessors but does something compeletly different. As the offsetof
calculation is clear in the context of extent buffer operations we can
remove it.
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.h

@@ -1973,11 +1973,6 @@ static inline void btrfs_set_header_backref_rev(struct extent_buffer *eb,
 	btrfs_set_header_flags(eb, flags);
 }
 
-static inline unsigned long btrfs_header_fsid(void)
-{
-	return offsetof(struct btrfs_header, fsid);
-}
-
 static inline unsigned long btrfs_header_chunk_tree_uuid(const struct extent_buffer *eb)
 {
 	return offsetof(struct btrfs_header, chunk_tree_uuid);

fs/btrfs/disk-io.c

@@ -541,7 +541,8 @@ static int csum_dirty_buffer(struct btrfs_fs_info *fs_info, struct page *page)
 		return -EUCLEAN;
 
 	ASSERT(memcmp_extent_buffer(eb, fs_info->fs_devices->metadata_uuid,
-			btrfs_header_fsid(), BTRFS_FSID_SIZE) == 0);
+				    offsetof(struct btrfs_header, fsid),
+				    BTRFS_FSID_SIZE) == 0);
 
 	if (csum_tree_block(eb, result))
 		return -EINVAL;
@@ -571,7 +572,8 @@ static int check_tree_block_fsid(struct extent_buffer *eb)
 	u8 fsid[BTRFS_FSID_SIZE];
 	int ret = 1;
 
-	read_extent_buffer(eb, fsid, btrfs_header_fsid(), BTRFS_FSID_SIZE);
+	read_extent_buffer(eb, fsid, offsetof(struct btrfs_header, fsid),
+			   BTRFS_FSID_SIZE);
 	while (fs_devices) {
 		u8 *metadata_uuid;