[MIPS] Workaround for a sparse warning in include/asm-mips/compat.h

Cast to a __user pointer via "unsigned long" to get rid of this warning:

include2/asm/compat.h:135:10: warning: cast adds address space to expression (<asn:1>)

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>