sched, sched_ext: Simplify dl_prio() case handling in sched_fork()

sched_fork() returns with -EAGAIN if dl_prio(@p). a7a9fc54 ("sched_ext:
Add boilerplate for extensible scheduler class") added scx_pre_fork() call
before it and then scx_cancel_fork() on the exit path. This is silly as the
dl_prio() block can just be moved above the scx_pre_fork() call.

Move the dl_prio() block above the scx_pre_fork() call and remove the now
unnecessary scx_cancel_fork() invocation.

Signed-off-by: Tejun Heo <tj@kernel.org>
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: David Vernet <void@manifault.com>