iflib: Detach tasks upon device registration failure
In some error paths we would fail to detach from the iflib taskqueue
groups. Also move the detach code into its own subroutine instead of
duplicating it.
Submitted by: Sai Rajesh Tallamraju <stallamr@netapp.com>
MFC after: 2 weeks
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D27342