vfs: fully hold vnodes in vnlru_free_locked

Currently the code only bumps holdcnt and clears the VI_FREE flag, not
performing actual vhold. Since the vnode is still visible elsewhere, a
potential new user can find it and incorrectly assume it is properly held.

Use vholdl instead to correctly hold the vnode. Another place recycling
(vlrureclaim) does this already.

Reviewed by: kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D21522