[scudo][standalone] Fix a race in the secondary release

Summary:
I tried to move the `madvise` calls outside of one of the secondary
mutexes, but this backfired. There is situation when a low release
interval is set combined with secondary pressure that leads to a race:
a thread can get a block from the cache, while another thread is
`madvise`'ing that block, resulting in a null header.

I changed the secondary race test so that this situation would be
triggered, and moved the release into the cache mutex scope.

Reviewers: cferris, pcc, eugenis, hctim, morehouse

Subscribers: jfb, #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D74072