fbnic: Cleanup handling of completions

There was an issue in that if we were to shutdown we could be left with
a completion in flight as the mailbox went away. To address that I have
added an fbnic_mbx_evict_all_cmpl function that is meant to essentially
create a "broken pipe" type response so that all callers will receive an
error indicating that the connection has been broken as a result of us
shutting down the mailbox.

Fixes: 378e5cc1 ("eth: fbnic: hwmon: Add completion infrastructure for firmware requests")
Signed-off-by: Alexander Duyck <alexanderduyck@fb.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://patch.msgid.link/174654720578.499179.380252598204530873.stgit@ahduyck-xeon-server.home.arpa


Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>