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>
Sponsored by: NetApp, Inc.

(cherry picked from commit 102540192c7195c99722869a11a48868e9116e91)