make request_module() killable

As Tetsuo pointed out, request_module() is very much unfriendly wrt OOM. It needs "a lot" of time/memory to finish while the caller is blocked in TASK_UNINTERRUPTIBLE.

Changes:

- improve the comments a bit

- reorder the changes so that 3/6 which changes kABI can be skipped, to simplify the backporting

- tried to test, seems to work

This series is orthogonal to kmod-avoid-deadlock-by-recursive-kmod-call.patch (which I still think should be replaced, I'll try to return to this later).

3/6 depends on usermodehelper-use-umh_wait_proc-consistently.patch

Many thanks to Tetsuo for the discussion.

Oleg.