btrfs: allow btrfs_submit_bio to split bios

Currently the I/O submitters have to split bios according to the chunk
stripe boundaries.  This leads to extra lookups in the extent trees and
a lot of boilerplate code.

To drop this requirement, split the bio when __btrfs_map_block returns a
mapping that is smaller than the requested size and keep a count of
pending bios in the original btrfs_bio so that the upper level
completion is only invoked when all clones have completed.

Based on a patch from Qu Wenruo.

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