ffs snapshots: pass valid args to vn_start_write

If the initial no-wait vn_start_write fails, we pass the old
PCATCH flag to the following sleepable vn_start_write.
Following a75d1ddd74, that flag should be V_PCATCH.

Sponsored by:		Dell Inc.
Differential Revision:	https://reviews.freebsd.org/D55512

sys/ufs/ffs/ffs_snapshot.c

@@ -282,7 +282,7 @@ restart:
 		NDFREE_PNBUF(&nd);
 		vput(nd.ni_dvp);
 		if ((error = vn_start_write(NULL, &wrtmp,
-		    V_XSLEEP | PCATCH)) != 0)
+		    V_XSLEEP | V_PCATCH)) != 0)
 			return (error);
 		goto restart;
 	}