The replacement of buf_pool.page_hash with a different type of
hash table in commit 5155a300 (MDEV-22871)
introduced a race condition with buffer pool resizing.

We have an execution trace where buf_pool.page_hash.array is changed
to point to something else while page_hash_latch::read_lock() is
executing. The same should also affect page_hash_latch::write_lock().

We fix the race condition by never resizing (and reallocating) the
buf_pool.page_hash. We assume that resizing the buffer pool is
a rare operation. Yes, there might be a performance regression if a
server is first started up with a tiny buffer pool, which is later
enlarged. In that case, the tiny buf_pool.page_hash.array could cause
increased use of the hash bucket lists. That problem can be worked
around by initially starting up the server with a larger buffer pool
and then shrinking that, until changing to a larger size again.

buf_pool_t::resize_hash(): Remove.

buf_pool_t::page_hash_table::lock(): Do not attempt to deal with
hash table resizing. If we really wanted that in a safe manner,
we would probably have to introduce a global rw-lock around the
operation, or at the very least, poll buf_pool.resizing, both of
which would be detrimental to performance.