r222015 introduced a new assertion that the size of a fixed-length sbuf
buffer is greater than 1.  This triggered panics in at least one spot in
the kernel (the MAC Framework) which passes non-negative, rather than >1
buffer sizes based on the size of a user buffer passed into a system
call.  While 0-size buffers aren't particularly useful, they also aren't
strictly incorrect, so loosen the assertion.

Discussed with:	phk
Spotted by:	pho + stress2


Index: subr_sbuf.c
===================================================================
--- subr_sbuf.c	(revision 224965)
+++ subr_sbuf.c	(working copy)
@@ -184,7 +184,7 @@
 	s->s_buf = buf;
 
 	if ((s->s_flags & SBUF_AUTOEXTEND) == 0) {
-		KASSERT(s->s_size > 1,
+		KASSERT(s->s_size >= 0,
 		    ("attempt to create a too small sbuf"));
 	}