m68k/mm: Port OOM changes to do_page_fault()

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 m68k.

Signed-off-by: Kautuk Consul <consul.kautuk@gmail.com>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>