nfsd: fix nfsdfs inode reference count leak

I don't understand this code well, but  I'm seeing a warning about a
still-referenced inode on unmount, and every other similar filesystem
does a dput() here.

Fixes: e8a79fb1

 ("nfsd: add nfsd/clients directory")
Signed-off-by: J. Bruce Fields <bfields@redhat.com>