unionfs: implement vnode-based cache lookup

unionfs uses a per-directory hashtable to cache subdirectory nodes.
Currently this hashtable is looked up using the directory name, but
since unionfs nodes aren't removed from the cache until they're
reclaimed, this poses some problems. For example, if a directory is
created on a unionfs mount shortly after deleting a previous directory
with the same path, the cache may end up reusing the node for the
previous directory, including its upper/lower FS vnodes. Operations
against those vnodes with then likely fail because the vnodes
represent deleted files; for example UFS will reject VOP_MKDIR()
against such a vnode because its effective link count is 0. This may
then manifest as e.g. mkdir(2) or open(2) returning ENOENT for an
attempt to create a file under the re-created directory.

While it would be possible to fix this by explicitly managing the
name-based cache during delete or rename operations, or by rejecting
cache hits if the underlying FS vnodes don't match those passed to
unionfs_nodeget(), it seems cleaner to instead hash the unionfs nodes
based on their underlying FS vnodes. Since unionfs prefers to operate
against the upper vnode if one is present, the lower vnode will only
be used for hashing as long as the upper vnode is NULL. This should
also make hashing faster by eliminating string traversal and using
the already-computed hash index stored in each vnode.

While here, fix a couple of other cache-related issues:

--Remove 8 bytes of unnecessary baggage from each unionfs node by

getting rid of the stored hash mask field.  The mask is knowable
at compile time.

--When a matching node is found in the cache, reference its vnode

using vrefl() while still holding the vnode interlock.  Previously
unionfs_nodeget() would vref() the vnode after the interlock was
dropped, but the vnode may be reclaimed during that window.  This
caused intermittent panics from vn_lock(9) during unionfs stress
testing.

Reviewed by: kib, markj
Tested by: pho
Differential Revision: https://reviews.freebsd.org/D32533