x86/pti: Fix boot warning from Global-bit setting

commit 231df823c4f04176f607afc4576c989895cff40e

The pageattr.c code attempts to process "faults" when it goes looking
for PTEs to change and finds non-present entries.  It allows these
faults in the linear map which is "expected to have holes", but
WARN()s about them elsewhere, like when called on the kernel image.

However, change_page_attr_clear() is now called on the kernel image in the
process of trying to clear the Global bit.

This trips the warning in __cpa_process_fault() if a non-present PTE is
encountered in the kernel image.  The "holes" in the kernel image result
from free_init_pages()'s use of set_memory_np().  These holes are totally
fine, and result from normal operation, just as they would be in the kernel
linear map.

Just silence the warning when holes in the kernel image are encountered.

Fixes: 39114b7a (x86/pti: Never implicitly clear _PAGE_GLOBAL for kernel image)
Reported-by: Mariusz Ceier <mceier@gmail.com>
Reported-by: Aaro Koskinen <aaro.koskinen@nokia.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Aaro Koskinen <aaro.koskinen@nokia.com>
Acked-by: Ingo Molnar <mingo@kernel.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Nadav Amit <namit@vmware.com>
Cc: Kees Cook <keescook@google.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: linux-mm@kvack.org
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Link: https://lkml.kernel.org/r/20180420222021.1C7D2B3F@viggo.jf.intel.com