libdtrace: Add vfs.d and define enum cache_fpl_status values there

[0mp]

Summary

Enum cache_fpl_status contains different status values that can be
observed with the vfs:fplookup:lookup:done probe.

MFC after: 1 week
Relnotes: yes

Test Plan

Copy vfs.d to /usr/lib/dtrace and run

vfs:fplookup:lookup:done /(int64_t)args[0]->ni_dirp < 0 && arg2 == CACHE_FPL_STATUS_HANDLED/ {
        trace(stringof(args[0]->ni_dirp));
}

[markj]

Please don't commit this. It's not necessary, and you're just hard-coding internal values from the kernel. The fplookup result status number is just an implementation detail and isn't part of any stable interface.

[0mp]

Great! Thanks for explaining, Mark :)