xfs: don't bother reporting blocks trimmed via FITRIM

Don't bother reporting the number of bytes that we "trimmed" because the
underlying storage isn't required to do anything(!) and failed discard
IOs aren't reported to the caller anyway.  It's not like userspace can
use the reported value for anything useful like adjusting the offset
parameter of the next call, and it's not like anyone ever wrote a
manpage about FITRIM's out parameters.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Tested-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>