Fix compat regression in process_vm_rw()

The removal of compat_process_vm_{readv,writev} didn't change
process_vm_rw(), which always assumes it's not doing a compat syscall.

Instead of passing in 'false' unconditionally for 'compat', make it
conditional on in_compat_syscall().

[ Both Al and Christoph point out that trying to access a 64-bit process
  from a 32-bit one cannot work anyway, and is likely better prohibited,
  but that's a separate issue    - Linus ]

Fixes: c3973b40 ("mm: remove compat_process_vm_{readv,writev}")
Reported-and-tested-by: NKyle Huey <me@kylehuey.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>
Acked-by: NAl Viro <viro@zeniv.linux.org.uk>
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/process_vm_access.c

@@ -273,7 +273,8 @@ static ssize_t process_vm_rw(pid_t pid,
 		return rc;
 	if (!iov_iter_count(&iter))
 		goto free_iov_l;
-	iov_r = iovec_from_user(rvec, riovcnt, UIO_FASTIOV, iovstack_r, false);
+	iov_r = iovec_from_user(rvec, riovcnt, UIO_FASTIOV, iovstack_r,
+				in_compat_syscall());
 	if (IS_ERR(iov_r)) {
 		rc = PTR_ERR(iov_r);
 		goto free_iov_l;