vf/remap: return the amount of bytes actually deduplicated

When using the FIDEDUPRANGE ioctl, in case of success the requested size
is returned. In some cases this might not be the actual amount of bytes
deduplicated.

This change modifies vfs_dedupe_file_range() to report the actual amount
of bytes deduplicated, instead of the requested amount.

Link: https://lore.kernel.org/linux-fsdevel/5548ef63-62f9-4f46-5793-03165ceccacc@tu-darmstadt.de/


Reported-by: Ansgar Lößer <ansgar.loesser@kom.tu-darmstadt.de>
Reported-by: Max Schlecht <max.schlecht@informatik.hu-berlin.de>
Reported-by: Björn Scheuermann <scheuermann@kom.tu-darmstadt.de>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Darrick J Wong <djwong@kernel.org>
Signed-off-by: Ansgar Lößer <ansgar.loesser@kom.tu-darmstadt.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>