nilfs2: fix potential bug in end_buffer_async_write
In the Linux kernel, the following vulnerability has been resolved:

nilfs2: fix potential bug in end_buffer_async_write

According to a syzbot report, end_buffer_async_write(), which handles the
completion of block device writes, may detect abnormal condition of the
buffer async_write flag and cause a BUG_ON failure when using nilfs2.

Nilfs2 itself does not use end_buffer_async_write().  But, the async_write
flag is now used as a marker by commit 7f42ec394156 ("nilfs2: fix issue
with race condition of competition between segments for dirty blocks") as
a means of resolving double list insertion of dirty blocks in
nilfs_lookup_dirty_data_buffers() and nilfs_lookup_node_buffers() and the
resulting crash.

This modification is safe as long as it is used for file data and b-tree
node blocks where the page caches are independent.  However, it was
irrelevant and redundant to also introduce async_write for segment summary
and super root blocks that share buffers with the backing device.  This
led to the possibility that the BUG_ON check in end_buffer_async_write
would fail as described above, if independent writebacks of the backing
device occurred in parallel.

The use of async_write for segment summary buffers has already been
removed in a previous change.

Fix this issue by removing the manipulation of the async_write flag for
the remaining super root block buffer.