arm64: Add get_user() type annotation on the !access_ok() path

Sparse reports "Using plain integer as NULL pointer" when the arm64
__get_user_error() assigns 0 to a pointer type. Use proper type
annotation.

Signed-of-by: Al Viro <viro@zeniv.linux.org.uk>
Reported-by: kbuild test robot <lkp@intel.com>
Link: http://lkml.kernel.org/r/20200522142321.GP23230@ZenIV.linux.org.uk
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>