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: Kyle Huey <me@kylehuey.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>