sparc64: Kill annoying warning when building compat_binfmt_elf.o

GCC warns because some tests against 32-bit values never evaluate to
true due to how TASK_SIZE is defined.

I always wanted to mimick powerpc's definition of TASK_SIZE, which
is simply TASK_SIZE_OF(current) and that also fixes the warning.

Signed-off-by: David S. Miller <davem@davemloft.net>