btrfs: split inode extref processing from __add_inode_ref() into a helper

The __add_inode_ref() function is quite big and with too much nesting, so
move the code that processes inode extrefs into a helper function, to make
the function easier to read and reduce the level of indentation too.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>