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>