sunrpc: only free unix grouplist after RCU settles

While the unix_gid object is rcu-freed, the group_info list that it
contains is not. Ensure that we only put the group list reference once
we are really freeing the unix_gid object.

Reported-by: Zhi Li <yieli@redhat.com>
Link: https://bugzilla.redhat.com/show_bug.cgi?id=2183056


Signed-off-by: Jeff Layton <jlayton@kernel.org>
Fixes: fd5d2f78 ("SUNRPC: Make server side AUTH_UNIX use lockless lookups")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>