x86: Move do_page_fault()'s error path under unlikely()

x86: Move do_page_fault()'s error path under unlikely()

Ingo suggested SIGKILL check should be moved into slowpath
function. This will reduce the page fault fastpath impact
of this recent commit:

  37b23e0525d3: x86,mm: make pagefault killable

Suggested-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: kamezawa.hiroyu@jp.fujitsu.com
Cc: minchan.kim@gmail.com
Cc: willy@linux.intel.com
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/4DDE0B5C.9050907@jp.fujitsu.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>