commit de942c9f (MDEV-15983)
introduced a race condition that we inadequately fixed in
commit 93b69825 (MDEV-16169).

Because fil_space_t::release() or fil_space_t::acquire() are
not protected by fil_system.mutex like their predecessors,
it is possible that stop_new_ops was set between the time
a thread checked fil_space_t::is_stopping() and invoked
fil_space_t::acquire().

In an execution trace, this happened in fil_system_t::keyrotate_next(),
causing an assertion failure in fil_delete_tablespace()
in the other thread that seeked to stop new operations.

We fix this bug by merging the flag fil_space_t::stop_new_ops
and the reference count fil_space_t::n_pending_ops into a
single word that is only being accessed by atomic memory operations.

fil_space_t::set_stopping(): Accessor for changing the state of
the former stop_new_ops flag.

fil_space_t::acquire(): Return whether the acquisition succeeded.
It would fail between set_stopping(true) and set_stopping(false).