iflib: Add registration failure injection points

[kbowling]

Summary

Add six device-scoped fail(9) points at the registration milestones
needed to exercise each unwind path. An exact, runtime-only device
selector prevents unrelated iflib devices from consuming an armed point.

Mark the points non-sleepable because registration holds the ifnet and
context locks. Document one-shot operation and bus-address reprobe so a
failed attach can be recovered without another kernel build.

MFC after:      2 weeks

[ziaee]

share/man/man4/iflib.4

251

I think this should be a Ss of DESCRIPTION or SYSCTL VARIABLES.

[gallatin]

sys/net/iflib.c

5289

is this assignment needed? How did err become non-zero? IFDI_ATTACH_PRE(ctx) must have set err to 0 in order to reach this code.
similar logic applies to all these err=0 assignments except the first.

I might consider making the label a macro arg so as to eliminate the if (err=0) goto label check after each macro invocation. That would make this less obtrusive.

[kbowling]

IFLIB_REGISTER_FAIL_POINT takes label param