arm64: prevent panic when using syscall mux + large arg call (mmap)

if the syscall muxes are used, up to two additional arguments
may be required.  This means that the 8 required for mmap increases
up to 10 (for __syscall).

Sponsored by:   Juniper Networks, Inc.

Author: jmgurney

sys/arm64/arm64/elf32_machdep.c

@@ -195,7 +195,7 @@ freebsd32_fetch_syscall_args(struct thread *td)
 	register_t *ap;
 	struct syscall_args *sa;
 	int error, i, nap, narg;
-	unsigned int args[4];
+	unsigned int args[6];
 
 	nap = 4;
 	p = td->td_proc;