add default_gfp() helper macro and use it in the new *alloc_obj() helpers

add default_gfp() helper macro and use it in the new *alloc_obj() helpers

Most simple allocations use GFP_KERNEL, and with the new allocation
helpers being introduced, let's just take advantage of that to simplify
that default case.

It's a numbers game:

    git grep 'alloc_obj(' |
	sed 's/.*\(GFP_[_A-Z]*\).*/\1/' |
	sort | uniq -c | sort -n | tail

shows that about 90% of all those new allocator instances just use that
standard GFP_KERNEL.

Those helpers are already macros, and we can easily just make it be the
default case when the gfp argument is missing.

And yes, we could do that for all the legacy interfaces too, but let's
keep it to just the new ones at least for now, since those all got
converted recently anyway, so this is not any "extra" noise outside of
that limited conversion.

And, in fact, I want to do this before doing the -rc1 release, exactly
so that we don't get extra merge conflicts.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>