nfsd: avoid ref leak in nfsd_open_local_fh()

In the Linux kernel, the following vulnerability has been resolved:

nfsd: avoid ref leak in nfsd_open_local_fh()

If two calls to nfsd_open_local_fh() race and both successfully call
nfsd_file_acquire_local(), they will both get an extra reference to the
net to accompany the file reference stored in *pnf.

One of them will fail to store (using xchg()) the file reference in
*pnf and will drop that reference but WON'T drop the accompanying
reference to the net. This leak means that when the nfs server is shut
down it will hang in nfsd_shutdown_net() waiting for
&nn->nfsd_net_free_done.

This patch adds the missing nfsd_net_put().