fs/compat_rw_copy_check_uvector: add missing compat_ptr call

A call to access_ok is missing a compat_ptr conversion.  Introduced with
b8373363 "compat: factor out
compat_rw_copy_check_uvector from compat_do_readv_writev"

fs/compat.c: In function 'compat_rw_copy_check_uvector':
fs/compat.c:629: warning: passing argument 1 of '__access_ok' makes pointer from integer without a cast

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>