PM / Hibernate: Rewrite unlock_system_sleep() to fix s2disk regression

Commit 33e638b9, "PM / Sleep: Use the freezer_count() functions in
[un]lock_system_sleep() APIs" introduced an undesirable change in the
behaviour of unlock_system_sleep() since freezer_count() internally calls
try_to_freeze() - which we don't need in unlock_system_sleep().

And commit bcda53fa, "PM / Sleep: Replace mutex_[un]lock(&pm_mutex) with
[un]lock_system_sleep()" made these APIs wide-spread. This caused a
regression in suspend-to-disk where snapshot_read() and snapshot_write()
were getting frozen due to the try_to_freeze embedded in
unlock_system_sleep(), since these functions were invoked when the freezing
condition was still in effect.

Fix this by rewriting unlock_system_sleep() by open-coding freezer_count()
and dropping the try_to_freeze() part. Not only will this fix the
regression but this will also ensure that the API only does what it is
intended to do, and nothing more, under the hood.

While at it, make the code more correct and robust by ensuring that the
PF_FREEZER_SKIP flag gets cleared with pm_mutex held, to avoid a race with
the freezer.

Also, to be on the safer side, open-code freezer_do_not_count() as well
(inside lock_system_sleep()), to ensure that any unrelated modification to
freezer[_do_not]_count() does not break things again!

Reported-and-tested-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>