sched_ext: Implement scx_bpf_dispatch[_vtime]_from_dsq()

sched_ext: Implement scx_bpf_dispatch[_vtime]_from_dsq()

Once a task is put into a DSQ, the allowed operations are fairly limited.
Tasks in the built-in local and global DSQs are executed automatically and,
ignoring dequeue, there is only one way a task in a user DSQ can be
manipulated - scx_bpf_consume() moves the first task to the dispatching
local DSQ. This inflexibility sometimes gets in the way and is an area where
multiple feature requests have been made.

Implement scx_bpf_dispatch[_vtime]_from_dsq(), which can be called during
DSQ iteration and can move the task to any DSQ - local DSQs, global DSQ and
user DSQs. The kfuncs can be called from ops.dispatch() and any BPF context
which dosen't hold a rq lock including BPF timers and SYSCALL programs.

This is an expansion of an earlier patch which only allowed moving into the
dispatching local DSQ:

  http://lkml.kernel.org/r/Zn4Cw4FDTmvXnhaf@slm.duckdns.org

v2: Remove @slice and @vtime from scx_bpf_dispatch_from_dsq[_vtime]() as
    they push scx_bpf_dispatch_from_dsq_vtime() over the kfunc argument
    count limit and often won't be needed anyway. Instead provide
    scx_bpf_dispatch_from_dsq_set_{slice|vtime}() kfuncs which can be called
    only when needed and override the specified parameter for the subsequent
    dispatch.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Daniel Hodges <hodges.daniel.scott@gmail.com>
Cc: David Vernet <void@manifault.com>
Cc: Changwoo Min <multics69@gmail.com>
Cc: Andrea Righi <andrea.righi@linux.dev>
Cc: Dan Schatzberg <schatzberg.dan@gmail.com>