vfs: remap helper should update destination inode metadata

Extend generic_remap_file_range_prep to handle inode metadata updates
when remapping into a file.  If the operation can possibly alter the
file contents, we must update the ctime and mtime and remove security
privileges, just like we do for regular file writes.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>