Silence warning and set WARNS=2.

Submitted by:	Mike Barcroft <mike@q9media.com>
Reviewed by:	md5(1)

usr.bin/lsvfs/Makefile

@@ -1,4 +1,5 @@
 # $FreeBSD$
 PROG=	lsvfs
+WARNS?=	2
 
 .include <bsd.prog.mk>

usr.bin/lsvfs/lsvfs.c

@@ -44,7 +44,7 @@ main(int argc, char **argv)
       }
     }
   } else {
-    while (ovfcp = getvfsent()) {
+    while ((ovfcp = getvfsent()) != NULL) {
       printf(FMT, ovfcp->vfc_name, ovfcp->vfc_refcount,
              fmt_flags(ovfcp->vfc_flags));
     }