nospec: Allow getting/setting on non-current task

nospec: Allow getting/setting on non-current task

Adjust arch_prctl_get/set_spec_ctrl() to operate on tasks other than
current.

This is needed both for /proc/$pid/status queries and for seccomp (since
thread-syncing can trigger seccomp in non-current threads).

OraBug: 28041771
CVE: CVE-2018-3639

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
(cherry picked from commit 7bbf1373e228840bb0295a2ca26d548ef37f448e)
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Mihai Carabas <mihai.carabas@oracle.com>
 Conflicts:
	arch/x86/kernel/cpu/bugs.c
	include/linux/nospec.h
	kernel/sys.c

Signed-off-by: Brian Maly <brian.maly@oracle.com>