nfsd: when reusing an existing repcache entry, unhash it first

The DRC code will attempt to reuse an existing, expired cache entry in
preference to allocating a new one. It'll then search the cache, and if
it gets a hit it'll then free the cache entry that it was going to
reuse.

The cache code doesn't unhash the entry that it's going to reuse
however, so it's possible for it end up designating an entry for reuse
and then subsequently freeing the same entry after it finds it.  This
leads it to a later use-after-free situation and usually some list
corruption warnings or an oops.

Fix this by simply unhashing the entry that we intend to reuse. That
will mean that it's not findable via a search and should prevent this
situation from occurring.

Cc: stable@vger.kernel.org # v3.10+
Reported-by: Christoph Hellwig <hch@infradead.org>
Reported-by: g. artim <gartim@gmail.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>