[IPV6] TCPMD5: Fix deleting key operation.

Due to the bug, refcnt for md5sig pool was leaked when
an user try to delete a key if we have more than one key.
In addition to the leakage, we returned incorrect return
result value for userspace.

This fix should close Bug #9418, reported by <ming-baini@163.com>.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>