livepatch: add locking to force and signal functions

klp_send_signals() and klp_force_transition() do not acquire klp_mutex,
because it seemed to be superfluous. A potential race in
klp_send_signals() was harmless and there was nothing in
klp_force_transition() which needed to be synchronized. That changed
with the addition of klp_forced variable during the review process.

There is a small window now, when klp_complete_transition() does not see
klp_forced set to true while all tasks have been already transitioned to
the target state. module_put() is called and the module can be removed.

Acquire klp_mutex in sysfs callback to prevent it. Do the same for the
signal sending just to be sure. There is no real downside to that.

Fixes: c99a2be790b07 ("livepatch: force transition to finish")
Fixes: 43347d56c8d9d ("livepatch: send a fake signal to all blocking tasks")
Reported-by: Jason Baron <jbaron@akamai.com>
Signed-off-by: Miroslav Benes <mbenes@suse.cz>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

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