btrfs: replace double boolean parameters of cow_file_range()

The function cow_file_range() has two boolean parameters.  Replace it
with a single @flags parameter, with two flags:

- COW_FILE_RANGE_NO_INLINE
- COW_FILE_RANGE_KEEP_LOCKED

And since we're here, also update the comments of cow_file_range() to
replace the old "page" usage with "folio".

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>