vfslist.c: initialize skipvfs variable

The function makevfslist is only called once in mount.c, but should
be save to be called more than once with different parameters.

The bin/df command links against this file, and will need this
possibility to allow -l and -t to be used together.

MFC after:	3 days

sbin/mount/vfslist.c

@@ -68,6 +68,7 @@ makevfslist(char *fslist)
 
 	if (fslist == NULL)
 		return (NULL);
+	skipvfs = 0;
 	if (fslist[0] == 'n' && fslist[1] == 'o') {
 		fslist += 2;
 		skipvfs = 1;