block: export disk_clear_zoned()

A previous commit split disk_set_zoned(..., bool) into not taking an
argument for whether to set or clear, and instead added
disk_clear_zoned() as the counterpart. However, that commit neglected
to export the new symbol, causing failures for modular drivers that
used it.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: d73e93b4 ("block: simplify disk_set_zoned")
Signed-off-by: Jens Axboe <axboe@kernel.dk>