sched_ext: Mark scx_sched_hash insecure_elasticity

sched_ext: Mark scx_sched_hash insecure_elasticity

scx_sched_hash is inserted into under scx_sched_lock (raw_spinlock_irq)
in scx_link_sched(). rhashtable's sync grow path calls get_random_u32()
and does a GFP_ATOMIC allocation; both acquire regular spinlocks, which
is unsafe under raw_spinlock_t. Set insecure_elasticity to skip the
sync grow.

v2:
- Dropped dsq_hash changes. Insertion is not under raw_spin_lock.

- Switched from no_sync_grow flag to insecure_elasticity.

Fixes: 25037af712eb ("sched_ext: Add rhashtable lookup for sub-schedulers")
Signed-off-by: Tejun Heo <tj@kernel.org>