sched_ext: Provide a sysfs enable_seq counter

As discussed during the distro-centric session within the sched_ext
Microconference at LPC 2024, introduce a sequence counter that is
incremented every time a BPF scheduler is loaded.

This feature can help distributions in diagnosing potential performance
regressions by identifying systems where users are running (or have ran)
custom BPF schedulers.

Example:

 arighi@virtme-ng~> cat /sys/kernel/sched_ext/enable_seq
 0
 arighi@virtme-ng~> sudo scx_simple
 local=1 global=0
 ^CEXIT: unregistered from user space
 arighi@virtme-ng~> cat /sys/kernel/sched_ext/enable_seq
 1

In this way user-space tools (such as Ubuntu's apport and similar) are
able to gather and include this information in bug reports.

Cc: Giovanni Gherdovich <[email protected]>
Cc: Kleber Sacilotto de Souza <[email protected]>
Cc: Marcelo Henrique Cerri <[email protected]>
Cc: Phil Auld <[email protected]>
Signed-off-by: Andrea Righi <[email protected]>
Signed-off-by: Tejun Heo <[email protected]>

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