unicore32/mm/fault.c: Port OOM changes to do_pf

Commit d065bd81
(mm: retry page fault when blocking on disk transfer) and
commit 37b23e05
(x86,mm: make pagefault killable)

The above commits introduced changes into the x86 pagefault handler
for making the page fault handler retryable as well as killable.

These changes reduce the mmap_sem hold time, which is crucial
during OOM killer invocation.

Port these changes to unicore32.

Signed-off-by: Kautuk Consul <consul.kautuk@gmail.com>
Acked-by: Guan Xuetao <gxt@mprc.pku.edu.cn>