tmpfs_destroy_vobject(): clear v_object under the object lock

Which allows tmpfs_pager_writecount_recalc() to reliably detect
reclaimed vnode and make its accesses to object->un_pager.swp.private
(== vp) safe against reclaim. Note that vnode instantiation already
assigns v_object under the object lock.

Reviewed by: markj
Reported and tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D45119