btrfs: extent-tree: Open-code process_func in __btrfs_mod_ref

The process_func function pointer is local to __btrfs_mod_ref() and
points to either btrfs_inc_extent_ref() or btrfs_free_extent().

Open code it to make later delayed ref refactor easier, so we can
refactor btrfs_inc_extent_ref() and btrfs_free_extent() in different
patches.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>