x86, mm: fault.c cleanup

Impact: cleanup, no code changed

Clean up various small details, which can be correctness checked
automatically:

 - tidy up the include file section
 - eliminate unnecessary includes
 - introduce show_signal_msg() to clean up code flow
 - standardize the code flow
 - standardize comments and other style details
 - more cleanups, pointed out by checkpatch

No code changed on either 32-bit nor 64-bit:

arch/x86/mm/fault.o:

   text	   data	    bss	    dec	    hex	filename
   4632	     32	     24	   4688	   1250	fault.o.before
   4632	     32	     24	   4688	   1250	fault.o.after

the md5 changed due to a change in a single instruction:

   2e8a8241e7f0d69706776a5a26c90bc0  fault.o.before.asm
   c5c3d36e725586eb74f0e10692f0193e  fault.o.after.asm

Because a __LINE__ reference in a WARN_ONCE() has changed.

On 32-bit a few stack offsets changed - no code size difference
nor any functionality difference.

Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: NIngo Molnar <mingo@elte.hu>