btrfs: export comp_keys() from ctree.c as btrfs_comp_keys()

Export comp_keys() out of ctree.c, as btrfs_comp_keys(), so that in a
later patch we can move out defrag specific code from ctree.c into
defrag.c.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>