btrfs: raid56: wait for I/O completion in submit_read_bios

In addition to setting up the end_io handler and submitting the bios in
submit_read_bios, also wait for them to be completed instead of waiting
for the completion manually in all three callers.

Rename submit_read_bios to submit_read_wait_bio_list to make it clear
it waits for the bios as well.

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>