mm: actually remap enough memory

For whatever reason, generic_access_phys() only remaps one page, but
actually allows to access arbitrary size. It's quite easy to trigger
large reads, like printing out large structure with gdb, which leads to
a crash. Fix it by remapping correct size.

Fixes: 28b2ee20c7cb ("access_process_vm device memory infrastructure")
Cc: stable@vger.kernel.org
Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>