exterror(9): add two helpers

The exterr_set_from() function sets current thread extended error from
the pre-filled struct kexterr.

The exterr_clear() function clears some struct kexterr.

Reviewed by:	mckusick
Sponsored by:	The FreeBSD Foundation
Differential revision:	https://reviews.freebsd.org/D53351

sys/kern/sys_generic.c

@@ -2345,3 +2345,22 @@ exterr_set(int eerror, int category, const char *mmsg, uintptr_t pp1,
 	}
 	return (eerror);
 }
+
+int
+exterr_set_from(const struct kexterr *ke)
+{
+	struct thread *td;
+
+	td = curthread;
+	if ((td->td_pflags2 & TDP2_UEXTERR) != 0) {
+		td->td_pflags2 |= TDP2_EXTERR;
+		td->td_kexterr = *ke;
+	}
+	return (td->td_kexterr.error);
+}
+
+void
+exterr_clear(struct kexterr *ke)
+{
+	memset(ke, 0, sizeof(*ke));
+}

sys/sys/exterrvar.h

@@ -69,6 +69,8 @@
 	_EXTERROR_MACRO(__VA_ARGS__, _SET_ERROR2, _SET_ERROR1,		\
 	    _SET_ERROR0)(__VA_ARGS__)
 
+void exterr_clear(struct kexterr *ke);
+int exterr_set_from(const struct kexterr *ke);
 int exterr_set(int eerror, int category, const char *mmsg, uintptr_t pp1,
     uintptr_t pp2, int line);
 int exterr_to_ue(struct thread *td, struct uexterror *ue);