btrfs: split delayed ref head initialization and addition

add_delayed_ref_head really performed 2 independent operations -
initialisting the ref head and adding it to a list. Now that the init
part is in a separate function let's complete the separation between
both operations. This results in a lot simpler interface for
add_delayed_ref_head since the function now deals solely with either
adding the newly initialised delayed ref head or merging it into an
existing delayed ref head. This results in vastly simplified function
signature since 5 arguments are dropped. The only other thing worth
mentioning is that due to this split the WARN_ON catching reinit of
existing. In this patch the condition is extended such that:

  qrecord && head_ref->qgroup_ref_root && head_ref->qgroup_reserved

is added. This is done because the two qgroup_* prefixed member are
set only if both ref_root and reserved are passed. So functionally
it's equivalent to the old WARN_ON and allows to remove the two args
from add_delayed_ref_head.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>