erofs: kill __submit_bio()

As Christoph pointed out [1], "
Why is there __submit_bio which really just obsfucates
what is going on?  Also why is __submit_bio using
bio_set_op_attrs instead of opencode it as the comment
right next to it asks you to? "

Let's use submit_bio directly instead.

[1] https://lore.kernel.org/r/20190830162812.GA10694@infradead.org/


Reported-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Link: https://lore.kernel.org/r/20190904020912.63925-18-gaoxiang25@huawei.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>