zloop: fix zone append check in zloop_rw()

While commit cf28f6f9 ("zloop: fail zone append operations that are
targeting full zones") added a check in zloop_rw() that a zone append is
not issued to a full zone, commit e3a96ca9 ("zloop: simplify checks
for writes to sequential zones") inadvertently removed the check to
verify that there is enough unwritten space in a zone for an incoming
zone append opration.

Re-add this check in zloop_rw() to make sure we do not write beyond the
end of a zone. Of note is that this same check is already present in the
function zloop_set_zone_append_sector() when ordered zone append is in
use.

Reported-by: Hans Holmberg <Hans.Holmberg@wdc.com>
Fixes: e3a96ca9 ("zloop: simplify checks for writes to sequential zones")
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Hans Holmberg <hans.holmberg@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>