injection_points: Fix incorrect spinlock acquisition

injection_points: Fix incorrect spinlock acquisition

Injection points created under injection_points_set_local() are cleaned
up by a shmem_exit() callback.  The spinlock used by the module would
be hold while calling InjectionPointDetach(), which is incorrect as
spinlocks should avoid external calls while hold.

This commit changes the shmem_exit() callback to detach the points in
three steps with the spinlock acquired twice, knowing that the
injection points should be around with the conditions related to them:
- Scans for the points to detach in a first loop, while holding the
spinlock.
- Detach them.
- Remove the registered conditions.

It is still possible for other processes to detach local points
concurrently of the callback.  I have wanted to restrict the detach, but
Noah has mentioned that he has in mind some cases that may require this
capability.  No tests in the tree based on injection points need that
currently.

Thinko in f587338dec87.

Reported-by: Noah Misch
Reviewed-by: Noah Misch
Discussion: https://postgr.es/m/20240501231214[email protected]