block: make BlockBackend->quiesce_counter atomic

block: make BlockBackend->quiesce_counter atomic

The main loop thread increments/decrements BlockBackend->quiesce_counter
when drained sections begin/end. The counter is read in the I/O code
path. Therefore this field is used to communicate between threads
without a lock.

Acquire/release are not necessary because the BlockBackend->in_flight
counter already uses sequentially consistent accesses and running I/O
requests hold that counter when blk_wait_while_drained() is called.
qatomic_read() can be used.

Use qatomic_fetch_inc()/qatomic_fetch_dec() for modifications even
though sequentially consistent atomic accesses are not strictly required
here. They are, however, nicer to read than multiple calls to
qatomic_read() and qatomic_set(). Since beginning and ending drain is
not a hot path the extra cost doesn't matter.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20230307210427.269214-2-stefanha@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>