kernel.h: Fix a typo in comment

kernel.h: Fix a typo in comment

Signed-off-by: Wei Wang <wvw@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Borislav Petkov <bp@suse.de>
Cc: Crt Mori <cmo@melexis.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: gregkh@linuxfoundation.org
Cc: wei.vince.wang@gmail.com
Link: https://lkml.kernel.org/lkml/20180424212241.16013-1-wvw@google.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>