LINUX 5.3.0: Use send_sig instead of force_sig

LINUX 5.3.0: Use send_sig instead of force_sig

Linux 5.3.0 commit 3cf5d076fb4d48979f382bc9452765bf8b79e740 "signal
Remove task parameter from force_sig" (part of siginfo-linus branch)
changes the parameters for the Linux kernel function force_sig. See LKML
thread starting at https://lkml.org/lkml/2019/5/22/1351

According to the LKML discussion and the above commit message force_sig
is only safe to deliver a synchronous signal to the current task. To
send a signal to another task, we're supposed to use send_sig instead,
which has been available since at least linux 2.6.12-rc12.

Currently, rx_knet calls force_sig to kill the rxk_ListenerTask.  With
the Linux 5.3.0 kernel, this module fails to compile due to the above
noted changes.

Replace the force_sig call with send_sig.  In order to use send_sig, the
rxk_listener thread must allow SIGKILL and during shutdown (umount)
SIGKILL must be unblocked for the rxk_listener thread.

Note that SIGKILL is initially blocked on rxk_listener and is only
unblocked when shutting down the thread.  Having the signal blocked is
sufficient to prevent unwanted signals from reaching the rxk_listener
thread during normal operation.

Reviewed-on: https://gerrit.openafs.org/13753
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 2b7af1243f46496c0b5973b3fa2a6396243f7613)

Change-Id: I6eb44311fbcc63adb6ebeb85a8e076922befd645
Reviewed-on: https://gerrit.openafs.org/13788
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>