Fix panic in zfs recv code. The last vnode (mountpoint's vnode) can have
0 usecount.

Reported by:	Thomas Backman <serenity@exscape.org>

Index: sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_vfsops.c
===================================================================
--- sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_vfsops.c	(wersja 195455)
+++ sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_vfsops.c	(kopia robocza)
@@ -917,7 +917,7 @@
 	for (zp = list_head(&zfsvfs->z_all_znodes); zp != NULL;
 	    zp = list_next(&zfsvfs->z_all_znodes, zp))
 		if (zp->z_dbuf) {
-			ASSERT(ZTOV(zp)->v_count > 0);
+			ASSERT(ZTOV(zp)->v_count >= 0);
 			zfs_znode_dmu_fini(zp);
 		}
 	mutex_exit(&zfsvfs->z_znodes_lock);