linux-user: declare sys_futex to have 6 arguments

sys_futex has 6 arguments, and all of these need to be copied. Fix incorrect
declaration in the mips_syscall_args array.

This change fixes the cases where the 5th and 6th arguments have non-zero
value and have importance. An example is a Linux implementation of
pthread_cond_wait() function.

Signed-off-by: Petar Jovanovic <petar.jovanovic@imgtec.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

linux-user/main.c

diff --git a/linux-user/main.c b/linux-user/main.c
index 7f15d3d..99c3b3f 100644
--- a/linux-user/main.c
+++ b/linux-user/main.c
@@ -2028,7 +2028,7 @@
 	MIPS_SYS(sys_fremovexattr, 2)	/* 4235 */
 	MIPS_SYS(sys_tkill	, 2)
 	MIPS_SYS(sys_sendfile64	, 5)
-	MIPS_SYS(sys_futex	, 2)
+	MIPS_SYS(sys_futex	, 6)
 	MIPS_SYS(sys_sched_setaffinity, 3)
 	MIPS_SYS(sys_sched_getaffinity, 3)	/* 4240 */
 	MIPS_SYS(sys_io_setup	, 2)