vfs_lookup(): re-check v_mountedhere on lock upgrade

The VV_CROSSLOCK handling logic may need to upgrade the covered
vnode lock depending upon the requirements of the filesystem into
which vfs_lookup() is walking.  This may involve transiently
dropping the lock, which can allow the target mount to be unmounted.

Tested by:	pho
Reviewed by:	markj
Differential Revision:	https://reviews.freebsd.org/D39272

sys/kern/vfs_lookup.c

@@ -1280,6 +1280,9 @@ vfs_lookup(struct nameidata *ndp)
 					error = ENOENT;
 					goto bad2;
 				}
+				if (dp->v_mountedhere != mp) {
+					continue;
+				}
 			} else
 				crosslkflags &= ~LK_NODDLKTREAT;
 		}