nios2: Port OOM changes to do_page_fault()

nios2: Port OOM changes to do_page_fault()

Commit d065bd810b6d ("mm: retry page fault when blocking on disk transfer") and
and commit 37b23e0525d3 ("x86,mm: make pagefault killable")

The above commits introduced changes into the nios2 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.

Signed-off-by: Ley Foon Tan <lftan@altera.com>