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