IB/uverbs: Set error code when fail to consume all flow_spec items

If the flow_spec items parsed count does not match the number of items
declared in the flow_attr command, or if not all bytes are used for
flow_spec items (eg. trailing garbage), a log message is reported and
the function leave through the error path. Unfortunately the error
code is currently not set.

This patch set error code to -EINVAL in such cases, so that the error
is reported to userspace instead of silently fail.

Link: http://marc.info/?i=cover.1386798254.git.ydroneaud@opteya.com

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