Track buddy's used ICM memory, and free it if all
of the buddy's memory bacame unused.
Do this only for STEs.
MODIFY_ACTION buddies are much smaller, so in case there
is a large amount of modify_header actions, which result
in large amount of MODIFY_ACTION buddies, doing this
cleanup during sync will result in performance hit while
not freeing significant amount of memory.

Signed-off-by: Yevgeny Kliteynik <kliteyn@nvidia.com>
Reviewed-by: Alex Vesker <valex@nvidia.com>
Reviewed-by: Mark Bloch <mbloch@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>