plog write for the dev t1 is always async
code to wait for async writes complete is not inside append_plog()

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
add inflight_ios_wq

in queue_current_bufffer()
repeating atomic_read and sleep for a while to wait for
the nr_inflight_ios to be zero. but this is not a good implementation.

this kind of looping elsewhere was disclaimed by the dm maintainers
before. So, simply use wait_event() like other part of dm

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>