x86: don't send SIGBUS for kernel page faults

x86: don't send SIGBUS for kernel page faults

Based on commit 96054569190bdec375fe824e48ca1f4e3b53dd36 upstream,
authored by Linus Torvalds.

This is my backport to the .27 kernel tree, hopefully preserving
the same functionality.

Original commit message:
	It's wrong for several reasons, but the most direct one is that the
	fault may be for the stack accesses to set up a previous SIGBUS.  When
	we have a kernel exception, the kernel exception handler does all the
	fixups, not some user-level signal handler.

	Even apart from the nested SIGBUS issue, it's also wrong to give out
	kernel fault addresses in the signal handler info block, or to send a
	SIGBUS when a system call already returns EFAULT.

Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>