ifnet(9): split ifc_alloc_unit() (should simplify code flow)

Allocate smallest unit number from pool via ifc_alloc_unit_next()
and exact unit number (if available) via ifc_alloc_unit_specific().

While here, address possible deadlock (mentioned in PR).

PR: 217401
MFC after: 5 days
Differential Revision: https://reviews.freebsd.org/D12551