btrfs: extent_io: Move the BUG_ON() in flush_write_bio() one level up

We have a BUG_ON() in flush_write_bio() to handle the return value of
submit_one_bio().

Move the BUG_ON() one level up to all its callers.

This patch will introduce temporary variable, @flush_ret to keep code
change minimal in this patch. That variable will be cleaned up when
enhancing the error handling later.

Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>