arm64/mm: fix page table check compile error for CONFIG_PGTABLE_LEVELS=2

If CONFIG_PGTABLE_LEVELS=2 and CONFIG_ARCH_SUPPORTS_PAGE_TABLE_CHECK=y,
then we trigger a compile error:

  error: implicit declaration of function 'pte_user_accessible_page'

Move the definition of page table check helper out of branch
CONFIG_PGTABLE_LEVELS > 2

Link: https://lkml.kernel.org/r/20220517074548.2227779-3-tongtiangen@huawei.com
Fixes: daf214c14dbe ("arm64/mm: enable ARCH_SUPPORTS_PAGE_TABLE_CHECK")
Signed-off-by: Tong Tiangen <tongtiangen@huawei.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Guohanjun <guohanjun@huawei.com>
Cc: Xie XiuQi <xiexiuqi@huawei.com>
Cc: kernel test robot <lkp@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>