btrfs: push all inline logic into cow_file_range

Currently we have a lot of duplicated checks of

if (start == 0 && fs_info->sectorsize == PAGE_SIZE)
	cow_file_range_inline();

Instead of duplicating this check everywhere, consolidate all of the
inline extent logic into a helper which documents all of the checks and
then use that helper inside of cow_file_range_inline().  With this we
can clean up all of the calls to either unconditionally call
cow_file_range_inline(), or at least reduce the checks we're doing
before we call cow_file_range_inline();

Reviewed-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>