cxgbe(4): Use ifaddr_event_ext instead of ifaddr_event for CLIP management.

The _ext event notification includes the address being added/removed and
that gives the driver an easy way to ignore non-IPv6 addresses. Remove
'tom' from the handler's name while here, it was moved out of t4_tom a
long time ago.

Sponsored by: Chelsio Communications

(cherry picked from commit f4ba035bca927e21f0f88ce56915523bafd573d9)