btrfs: raid56: fold recover_assemble_read_bios into recover_rbio

There is very little extra code in recover_rbio, and a large part of it
is the superfluous extra cleanup of the bio list.  Merge the two
functions, and only clean up the bio list after it has been added to
but before it has been emptied again by submit_read_wait_bio_list.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>