MFC the following revisions per request from gleb@:

MFC the following revisions per request from gleb@:

r197953: Add locking around access to parent node, and bail out when
the parent node is already freed rather than panicking the system.

r227822: Avoid panics from recursive rename operations. Not a perfect
patch but good enough for now.

r232959: Don't enforce LK_RETRY to get existing vnode in
tmpfs_alloc_vp().

Doomed vnode is hardly of any use here, besides all callers handle error
case. vfs_hash_get() does the same.

Don't mess with vnode holdcount, vget() takes care of it already.

r232960: Prevent tmpfs_rename() deadlock in a way similar to UFS

Unlock vnodes and try to lock them one by one. Relookup fvp and tvp.

git-svn-id: svn://svn.freebsd.org/base/stable/8@238553 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f