sparc: Fix user_addr_max() definition.

We need to use TASK_SIZE because for 64-bit tasks the value
of STACK_TOP actually sits in the middle of the address space
so we'll get false-negatives.

Adjust the TASK_SIZE definition on sparc64 to accomodate this,
in the context in which user_addr_max() is used we have the
test_thread_flag() definition available but not the one for
test_tsk_thread_flag().

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