if_vmove: return proper error status

if_vmove can fail if it lost a race and the vnet's already been moved. The
callers (and their callers) can generally cope with this, but right now
success is assumed. Plumb out the ENOENT from if_detach_internal if it
happens so that the error's properly reported to userland.

Reviewed by: bz, kp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D22780