BACKPORT: erofs: lazily initialize per-CPU workers and CPU hotplug hooks

Currently, when EROFS is built with per-CPU workers, the workers are
started and CPU hotplug hooks are registered during module initialization.
This leads to unnecessary worker start/stop cycles during CPU hotplug
events, particularly on Android devices that frequently suspend and resume.

This change defers the initialization of per-CPU workers and the
registration of CPU hotplug hooks until the first EROFS mount. This
ensures that these resources are only allocated and managed when EROFS is
actually in use.

The tear down of per-CPU workers and unregistration of CPU hotplug hooks
still occurs during z_erofs_exit_subsystem(), but only if they were
initialized.

Bug: 382400420
Change-Id: I895b5350e357c2165d44f85cf7a3811e0e48f1b0
Signed-off-by: Sandeep Dhavale <dhavale@google.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Link: https://lore.kernel.org/r/20250506225743.308517-1-dhavale@google.com


Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
(cherry picked from commit 12bf25d1)
[dhavale: erofs_init_managed_cache() was renamed upstream as
z_erofs_init_super() so the changes are applied to
erofs_init_managed_cache()]