sched_ext: Fix incorrect autogroup migration detection

In the Linux kernel, the following vulnerability has been resolved:

sched_ext: Fix incorrect autogroup migration detection

scx_move_task() is called from sched_move_task() and tells the BPF scheduler

that cgroup migration is being committed. sched_move_task() is used by both

cgroup and autogroup migrations and scx_move_task() tried to filter out

autogroup migrations by testing the destination cgroup and PF_EXITING but

this is not enough. In fact, without explicitly tagging the thread which is

doing the cgroup migration, there is no good way to tell apart

scx_move_task() invocations for racing migration to the root cgroup and an

autogroup migration.

This led to scx_move_task() incorrectly ignoring a migration from non-root

cgroup to an autogroup of the root cgroup triggering the following warning:

WARNING: CPU: 7 PID: 1 at kernel/sched/ext.c:3725 scx_cgroup_can_attach+0x196/0x340

...

Call Trace:

<TASK>

cgroup_migrate_execute+0x5b1/0x700

cgroup_attach_task+0x296/0x400

__cgroup_procs_write+0x128/0x140

cgroup_procs_write+0x17/0x30

kernfs_fop_write_iter+0x141/0x1f0

vfs_write+0x31d/0x4a0

__x64_sys_write+0x72/0xf0

do_syscall_64+0x82/0x160

entry_SYSCALL_64_after_hwframe+0x76/0x7e

Fix it by adding an argument to sched_move_task() that indicates whether the

moving is for a cgroup or autogroup migration. After the change,

scx_move_task() is called only for cgroup migrations and renamed to

scx_cgroup_move_task().