nfsd4: fix double free in nfsd4_do_async_copy()

This frees "copy->nf_src" before and again after the goto.

Fixes: ce0887ac ("NFSD add nfs4 inter ssc to nfsd4_copy")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Olga Kornievskaia <kolga@netapp.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>