xtensa/mm/fault.c: port OOM changes to do_page_fault

xtensa/mm/fault.c: port OOM changes to do_page_fault

Commits a99d657f79de ("mm: retry page fault when blocking on disk
transfer") and 64587d42be2d ("x86,mm: make pagefault killable")
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 xtensa.

Signed-off-by: Kautuk Consul <consul.kautuk@gmail.com>
Acked-by: Chris Zankel <chris@zankel.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>