zbd: handle conventional start zone in zbd_convert_to_open_zone()

zbd: handle conventional start zone in zbd_convert_to_open_zone()

At the beginning of zbd_convert_to_open_zone() function, a zone
is picked in semi-random manner to become a candidate zone for
redirecting the incoming write. In some circumstances, such as
unlimited MaxOpen or i/o range that spans the boundary between
conventional and sequential zones, a conventional zone may be
selected.

This may create problems in the subsequent for (;;) loop in the
same function. Failed assertions were observed during the execution
of newly introduced test #51 that showed that the code in that loop
was trying to lock and unlock conventional zones.

Check if the zone which has been initially picked is conventional.
If yes, force the zone selection to be re-tried until a sequential
zone is selected for further processing.

Signed-off-by: Dmitry Fomichev <dmitry.fomichev@wdc.com>
Reviewed-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>