btrfs: raid56: introduce the main entrance for RMW path

The new entrance will be called rmw_rbio(), it will have a streamlined
workflow by using submit-and-wait method.

Thus there will be no weird jumps between tons of functions, thus way
more reader friendly, and will make later expansion easier, as it's now
a straight workflow, the timing is way more clear.

Unfortunately we can not yet migrate the RMW path to use this new
entrance as we still need extra work to address the plug and
unlock_stripe() function.

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