IB/uverbs: Check comp_mask in destroy_flow

Just like the check added to create_flow in 22878dbc ("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.

[1] http://marc.info/?i=40175eda10d670d098204da6aa4c327a0171ae5f.1381510045.git.ydroneaud@opteya.com
[2] http://marc.info/?i=cover.1381510045.git.ydroneaud@opteya.com
[3] http://marc.info/?i=cover.1383773832.git.ydroneaud@opteya.com

Cc: Matan Barak <matanb@mellanox.com>
Link: http://marc.info/?i=cover.1386798254.git.ydroneaud@opteya.com

>
Signed-off-by: Yann Droneaud <ydroneaud@opteya.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>