nfp: flower: add missing clean up call to avoid memory leaks

nfp_flower_metadata_cleanup() is defined but never invoked,
not calling it will cause us to leak mask and statistics
queue memory on the host.

Fixes: 43f84b72 ("nfp: add metadata to each flow offload")
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>