btrfs: provide enumeration for __merge_refs mode argument

Replace hardcoded numeric values for __merge_refs 'mode' argument
with descriptive constants.

Signed-off-by: Edmund Nadolski <enadolski@suse.com>
Reviewed-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>