btrfs: pass the extent map tree's inode to add_extent_mapping()

Extent maps are always added to an inode's extent map tree, so there's no
need to pass the extent map tree explicitly to add_extent_mapping().

In order to facilitate an upcoming change that adds a shrinker for extent
maps, change add_extent_mapping() to receive the inode instead of its
extent map tree.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>