lib/vsprintf.c: kptr_restrict: fix pK-error in SysRq show-all-timers(Q)

When using ALT+SysRq+Q all the pointers are replaced with "pK-error" like
this:

	[23153.208033]   .base:               pK-error

with echo h > /proc/sysrq-trigger it works:

	[23107.776363]   .base:       ffff88023e60d540

The intent behind this behavior was to return "pK-error" in cases where
the %pK format specifier was used in interrupt context, because the
CAP_SYSLOG check wouldn't be meaningful.  Clearly this should only apply
when kptr_restrict is actually enabled though.

Reported-by: Stevie Trujillo <stevie.trujillo@gmail.com>
Signed-off-by: Dan Rosenberg <dan.j.rosenberg@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>