btrfs: raid56: prepare data checksums for later RMW verification

This is for later data checksum verification at RMW time.

This patch will try to allocate the needed memory for a locked rbio if
the rbio is for data exclusively (we don't want to handle mixed bg yet).
The memory will be released when the rbio is finished.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>