um/kernel/trap.c: port OOM changes to handle_page_fault()

um/kernel/trap.c: port OOM changes to handle_page_fault()

Commit d065bd810b6deb6 ("mm: retry page fault when blocking on disk
transfer") and commit 37b23e0525 ("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 um.

Signed-off-by: Kautuk Consul <consul.kautuk@gmail.com>
Cc: Jeff Dike <jdike@addtoit.com>
Cc: Richard Weinberger <richard@nod.at>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>