BACKPORT: FROMGIT: Mark anonymous struct field of 'struct vm_fault' as 'const'

The fields of this struct are only ever read after being initialised, so
mark it 'const' before somebody tries to modify it again. GCC will then
complain (with an error) about modification of these fields after they
have been initialised, although LLVM currently allows them without even
a warning:

https://bugs.llvm.org/show_bug.cgi?id=48755



Hopefully, future versions of LLVM will emit a warning.

Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Will Deacon <will@kernel.org>
Change-Id: I46e2de1252751b863442383eac22090c4b4606c0
Bug: 171278850
(cherry picked from commit 5857c920
https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git/log/?h=for-next/faultaround

)
[vinmenon: changes for speculative page fault]
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>