nfsd: return correct openowner when there is a race to put one in the hash

alloc_init_open_stateowner can return an already freed entry if there is
a race to put openowners in the hashtable.

In commit 7ffb5880, we changed it so that we allocate and initialize
an openowner, and then check to see if a matching one got stuffed into
the hashtable in the meantime. If it did, then we free the one we just
allocated and take a reference on the one already there. There is a bug
here though. The code will then return the pointer to the one that was
allocated (and has now been freed).

This wasn't evident before as this race almost never occurred. The Linux
kernel client used to serialize requests for a single openowner.  That
has changed now with v4.0 kernels, and this race can now easily occur.

Fixes: 7ffb5880
Cc: <stable@vger.kernel.org> # v3.17+
Cc: Trond Myklebust <trond.myklebust@primarydata.com>
Reported-by: Christoph Hellwig <hch@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jeff Layton <jeff.layton@primarydata.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>