NFSD: Using free_conn free connection

commit 3f42d2c428c724212c5f4249daea97e254eb0546 upstream.

Connection from alloc_conn must be freed through free_conn,
otherwise, the reference of svc_xprt will never be put.

Signed-off-by: Kinglong Mee <kinglongmee@gmail.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Willy Tarreau <w@1wt.eu>