ifnet(9): split ifc_alloc_unit() (should make code a bit cleaner).

[avos]

Summary

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

While here, fix potential endless loop (*unit < 0 -> alloc_unr() succeeds but ifunit() fails -> (*unit)++ and then alloc_unr_specific() fails ->(*unit) is overridden with return result (-1) and then used instead of previous unit number).

[glebius]

Does this fix this bug? https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=217401

[avos]

Yes (but without reassigning *unit - it will not change after alloc_unr_specific() anyway).

[glebius]

Can you please grab the PR? :)