sunrpc: fix memory leak in unix_gid cache.

When we look up an entry in the uid->gidlist cache, we take
a reference to the content but don't drop the reference to the
cache entry.  So it never gets freed.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>