sfxge(4): check buffer size for hash flags

[arybchik]

Summary

The efx_rx_scale_hash_flags_get interface is unsafe, as it does not
have an argument for the size of the output buffer used to return
the flags. While the only caller currently supplies a sufficiently
large buffer, this should be checked at runtime to avoid writing
past the end of the buffer.

Submitted by: Ivan Malov <ivan.malov at oktetlabs.ru>
Sponsored by: Solarflare Communications, Inc.

Test Plan

amd64 tinderbox kernel builds tested
driver loaded/unloaded, interfaces up/down, traffic works fine