In preparation for const-ifying the anonymous struct field of
'struct vm_fault', ensure that it is initialised using designated
initialisers.

Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: NNick Desaulniers <ndesaulniers@google.com>
Signed-off-by: NWill Deacon <will@kernel.org>