blkdev: Simple cleanup in blkdev_issue_zeroout()

In blkdev_issue_zeroout() we are submitting regular WRITE bios, so we do
not need to check for -EOPNOTSUPP specifically in case of error. Also
there is no need to have label submit: because there is no way to jump
out from the while cycle without an error and we really want to exit,
rather than try again. And also remove the check for (sz == 0) since at
that point sz can never be zero.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
CC: Dmitry Monakhov <dmonakhov@openvz.org>
CC: Jens Axboe <jaxboe@fusionio.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>