MIPS: signal.c: Fix an invalid cast in ISA mode bit handling

Fix:

arch/mips/kernel/signal.c: In function 'handle_signal':
arch/mips/kernel/signal.c:533:21: error: cast from pointer to integer of different size [-Werror=pointer-to-int-cast]
  unsigned int tmp = (unsigned int)current->mm->context.vdso;
                     ^
arch/mips/kernel/signal.c:536:9: error: cast to pointer from integer of different size [-Werror=int-to-pointer-cast]
  vdso = (void *)tmp;
         ^
cc1: all warnings being treated as errors

when building a 64-bit kernel.

This is not really a supported configuration, but the cast is wrong
either way, Linux makes the assumption that sizeof(void *) equals
sizeof(unsigned long) and therefore the latter type is expected to be
used where integer operations have to be applied to pointers for some
reason.

Signed-off-by: Maciej W. Rozycki <macro@codesourcery.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/8480/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>