btrfs: merge btrfs_folio_unlock_writer() into btrfs_folio_end_writer_lock()

The function btrfs_folio_unlock_writer() is already calling
btrfs_folio_end_writer_lock() to do the heavy lifting work, the only
missing 0 writer check.

Thus there is no need to keep two different functions, move the 0 writer
check into btrfs_folio_end_writer_lock(), and remove
btrfs_folio_unlock_writer().

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>