xfs_io: actually check copy file range helper return values

xfs_io: actually check copy file range helper return values

We need to check the return value of copy_src_filesize and
copy_dst_truncate because either could return -1 due to fstat/ftruncate
failure.

Fixes: 628e112afdd98c5 ("xfs_io: implement 'copy_range' command")
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>