erofs: avoid unnecessary variable `err'

variable `err' in z_erofs_submit_queue() isn't useful
here, remove it instead.

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Link: https://lore.kernel.org/r/20200919072730.24989-1-hsiangkao@redhat.com


Signed-off-by: Gao Xiang <hsiangkao@redhat.com>