[SCTP]: Unmap v4mapped addresses during SCTP_BINDX_REM_ADDR operation.

During the sctp_bindx() call to add additional addresses to the
endpoint, any v4mapped addresses are converted and stored as regular
v4 addresses.  However, when trying to remove these addresses, the
v4mapped addresses are not converted and the operation fails.  This
patch unmaps the addresses on during the remove operation as well.

Signed-off-by: Paolo Galtieri <pgaltieri@mvista.com>
Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>