MIPS: O32: Do not handle require 32 bytes from the stack to be readable.

Commit 46e12c07

 (MIPS: O32 / 32-bit:
Always copy 4 stack arguments.) change the O32 syscall handler to always
load four arguments from the userspace stack even for syscalls that
require fewer or no arguments to be copied.  This removes a large table
from kernel space and need to maintain it.  It appeared that it was ok
the implementation chosen requires 16 bytes of readable stack space
above the user stack pointer.

Turned out a few threading implementations munmap the user stack before
the thread exits resulting in errors due to the unreadable stack.

We now treat any failed load as a if the loaded value was zero and let
the actual syscall deal with the situation.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>