Commit ae870a68 ("arm64/mm: Convert to using
lock_mm_and_find_vma()") made do_page_fault() to use 'vma' even if
CONFIG_PER_VMA_LOCK is not defined, but the declaration is still in the
ifdef.

As a result, building kernel without the config fails with undeclared
variable error as below:

    arch/arm64/mm/fault.c: In function 'do_page_fault':
    arch/arm64/mm/fault.c:624:2: error: 'vma' undeclared (first use in this function); did you mean 'vmap'?
      624 |  vma = lock_mm_and_find_vma(mm, addr, regs);
          |  ^~~
          |  vmap

Fix it by moving the declaration out of the ifdef.

Fixes: ae870a68 ("arm64/mm: Convert to using lock_mm_and_find_vma()")
Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>