LinuxKPI: add default_gfp()

[bz]

Summary

Various new allocation macros can take an optional gfp_t argument.
If the argument is not given we need to set the GFP_KERNEL default.
While this is only internally used and I initialy called it differently,
should this spread elsewhere having the same name as in Linux will be
good.

Sponsored by: The FreeBSD Foundaton
MFC after: 3 days

[dumbbell]

Why name it __lkpi_default_gfp() and not __default_gfp() like Linux? I'm sure some drivers will use it if it's available :)

[bz]

In D56392#1291580, @dumbbell wrote:

Why name it __lkpi_default_gfp() and not __default_gfp() like Linux? I'm sure some drivers will use it if it's available :)

I would hope not.

I don't know the name of that thing in Linux either. I just prefixed it with lkpi for the internal part not supposed to be public KPI and added __ given it's macro.
I was silly enough initially to only use ``(_arg_or_default, ...) without the _discard only to be reminded that that gives you errors if you do not pass an argument in given ##VA_ARGS works on the "trailing" comma.

I'll rename it if that's easier.

[bz]

Ranme "internal" part of the function to __default_gfp as suggested by @dumbbell to match Linux.

[dumbbell]

I agree it looks like a private API and I also hope it's not used. But I already saw functions and macros starting with underscores being used anyway...