cxgbe(4): Fix virtual interface reattach.

Replace the DOOMED flag with a transient DETACHING flag that is cleared
when VI is detached. This fixes VI reattach when only the VI and not
the parent nexus is detached. The old flag was never cleared and
prevented subsequent synch op's related to the VI.

PR: 275260
Reviewed by: jhb
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D43287
Sponsored by: Chelsio Communications