nfsd: fix fi_delegees leak when fi_had_conflict returns true

Currently, nfs4_set_delegation takes a reference to an existing
delegation and then checks to see if there is a conflict. If there is
one, then it doesn't release that reference.

Change the code to take the reference after the check and only if there
is no conflict.

Signed-off-by: Jeff Layton <jlayton@primarydata.com>
Cc: stable@vger.kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>