sched/mmcid: Remove pointless preempt guard

sched/mmcid: Remove pointless preempt guard

This is a leftover from the early versions of this function where it could
be invoked without mm::mm_cid::lock held.

Remove it and add lockdep asserts instead.

Fixes: 653fda7ae73d ("sched/mmcid: Switch over to the new mechanism")
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260310202526.116363613@kernel.org