Use PF_MEMALLOC_NOIO

Linux 3.9.0 introduced a per-thread PF_MEMALLOC_NOIO flag that almost
perfectly matches the PF_NOFS flag previously implemented by repurposing
the PF_MUTEX_TESTER when CONFIG_RT_MUTEX_TESTER was set. Lets switch to
it.

Signed-off-by: Richard Yao <ryao@gentoo.org>