fuse: copy_file_range needs to strip setuid bits and update timestamps

Like ->write_iter(), we update mtime and strip setuid of dst file before
copy and like ->read_iter(), we update atime of src file after copy.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Acked-by: Miklos Szeredi <miklos@szeredi.hu>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>