Just like the check added to create_flow in 22878dbc9173 ("IB/core:
Better checking of userspace values for receive flow steering"),
comp_mask must be checked in destroy_flow too.
Since only empty comp_mask is currently supported, any other value
must be rejected.
This check was silently added in a previous patch[1] to move comp_mask
in extended command header, part of previous patchset[2] against
create/destroy_flow uverbs. The idea of moving comp_mask to the header
was discarded for the final patchset[3].
Unfortunately the check added in destroy_flow uverb was not integrated
in the final patchset.