[Drbd-dev] Get drbd-9.0 branch building with Linux 5.9 and 5.10

[Michael D Labriola]

Hey, everyone! I had the chance to try building the latest drbd-9.0
code against the 5.9 and 5.10 kernels this afternoon. Things almost
built, but not quite.  I made a series of commits to get things
compiling.  Most of it is trivial compat test changes and following
along w/ what's already been done in the mainline drbd8 module to get
it ready for 5.10.

I did not create a compat test for the switch from
sched_setscheduler() to sched_set_fifo_low() yet.  Mostly, because
I've never done that and have to wrap my head around how to add
another test. ;-)

I tried submitting a pull request via github, but got the auto-close
message.  So, here's take two.