zloop: simplify checks for writes to sequential zones

zloop: simplify checks for writes to sequential zones

The function zloop_rw() already checks early that a request is fully
contained within the target zone. So this check does not need to be done
again for regular writes to sequential zones. Furthermore, since zone
append operations are always directed to the zone write pointer
location, we do not need to check for their alignment to that value
after setting it. So turn the "if" checking the write pointer alignment
into an "else if".

While at it, improve the comment describing the write pointer
modification and how this value is corrected in case of error.

Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>