nvme: fix initialization of the zone bitmaps

The removal of the ->revalidate_disk method broke the initialization of
the zone bitmaps, as nvme_revalidate_disk now never gets called during
initialization.

Move the zone related code from nvme_revalidate_disk into a new helper in
zns.c, and call it from nvme_alloc_ns in addition to nvme_validate_ns to
ensure the zone bitmaps are initialized during probe.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>