arm64: implement TASK_SIZE_OF

include/linux/sched.h implements TASK_SIZE_OF as TASK_SIZE if it
is not set by the architecture headers.  TASK_SIZE uses the
current task to determine the size of the virtual address space.
On a 64-bit kernel this will cause reading /proc/pid/pagemap of a
64-bit process from a 32-bit process to return EOF when it reads
past 0xffffffff.

Implement TASK_SIZE_OF exactly the same as TASK_SIZE with
test_tsk_thread_flag instead of test_thread_flag.

Cc: stable@vger.kernel.org
Signed-off-by: Colin Cross <ccross@android.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>