cifs: fix allocation in cifs_write_allocate_pages

The gfp flags are currently set to __GPF_HIGHMEM, which doesn't allow
for any reclaim. Make this more resilient by or'ing that with
GFP_KERNEL. Also, get rid of the goto and unify the exit codepath.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Acked-by: Pavel Shilovsky <piastry@etersoft.ru>