PM: suspend: clean up redundant filesystems_freeze/thaw() handling

The recently introduced support for freezing filesystems during system
suspend included calls to filesystems_freeze() in both suspend_prepare()
and enter_state(), as well as calls to filesystems_thaw() in both
suspend_finish() and the Unlock path in enter_state(). These are
redundant.

Moreover, calling filesystems_freeze() twice, from both suspend_prepare()
and enter_state(), leads to a black screen and makes the system unable
to resume in some cases.

Address this as follows:

- filesystems_freeze() is already called in suspend_prepare(), which
is the proper and consistent place to handle pre-suspend operations.
The second call in enter_state() is unnecessary and so remove it.

- filesystems_thaw() is invoked in suspend_finish(), which covers
successful suspend/resume paths. In the failure case, add a call
to filesystems_thaw() only when needed, avoiding the duplicate call
in the general Unlock path.

This change simplifies the suspend code and avoids repeated freeze/thaw
calls, while preserving correct ordering and behavior.

Fixes: eacfbf74196f ("power: freeze filesystems during suspend/resume")
Signed-off-by: Zihuan Zhang <zhangzihuan@kylinos.cn>
Link: https://patch.msgid.link/20250712030824.81474-1-zhangzihuan@kylinos.cn
[ rjw: Changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>