sbuf: make assertion message a lot more useable

Saying that we are called with an (un)finished or corrupted sbuf
is like saying "error" (though [un]finished is an extra hint).

Add the pointer to the sbuf so one could easily check it,
add the flags we are checking so one could see quickly about
a possible state mismatch.

Given we already dereference the pointer in the KASSERT there is no
extra harm adding it to the message.

Found while:	testing lindebugfs changes
Sponsored by:	The FreeBSD Foundation
MFC after:	3 days
Reviewed by:	jhb
Differential Revision: https://reviews.freebsd.org/D55919

sys/kern/subr_sbuf.c

@@ -124,8 +124,8 @@ _assert_sbuf_state(const char *fun, struct sbuf *s, int state)
 {
 
 	KASSERT((s->s_flags & SBUF_FINISHED) == state,
-	    ("%s called with %sfinished or corrupt sbuf", fun,
-	    (state ? "un" : "")));
+	    ("%s called with %sfinished or corrupt sbuf %p { s_flags %#010x }, "
+		"state %#010x", fun, (state ? "un" : ""), s, s->s_flags, state));
 }
 
 #define	assert_sbuf_integrity(s) _assert_sbuf_integrity(__func__, (s))