modules: bzero the modspecific_t

Per https://reviews.llvm.org/D68115, only the first field is
zero-initialized, meanwhile other fields are undef.

The pattern can be observed on clang as well, that when
-ftrivial-auto-var-init=pattern is specified 0xaa is filled for
non-active fields, otherwise they are zero-initialized.
Technically both are acceptable when using clang. However it
would be good to simply bzero the modspecific_t in such case to
be strict to the standard.

MFC with:	2cab2d43b8
MFC after:	1 day
Sponsored by:	Juniper Networks, Inc.
Reviewed by:	delphij
Differential Revision:	https://reviews.freebsd.org/D40830

sys/kern/kern_syscalls.c

@@ -173,9 +173,10 @@ kern_syscall_module_handler(struct sysent *sysents, struct module *mod,
     int what, void *arg)
 {
 	struct syscall_module_data *data = arg;
-	modspecific_t ms = { 0 };
+	modspecific_t ms;
 	int error;
 
+	bzero(&ms, sizeof(ms));
 	switch (what) {
 	case MOD_LOAD:
 		error = kern_syscall_register(sysents, data->offset,