dm ioctl: remove PF_MEMALLOC

When allocating memory for the userspace ioctl data, set some
appropriate GPF flags directly instead of using PF_MEMALLOC.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>