btrfs: move __btrfs_bio_end_io() code into its single caller

The __btrfs_bio_end_io() helper is trivial and has a single caller, so
there's no point in having a dedicated helper function. Further the double
underscore prefix in the name is discouraged. So get rid of it and move
its code into the caller (btrfs_bio_end_io()).

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