ionic: make all rx_mode work threadsafe

Move the bulk of the code from ionic_set_rx_mode(), which
can be called from atomic context, into ionic_lif_rx_mode()
which is a safe context.

A call from the stack will get pushed off into a work thread,
but it is also possible to simultaneously have a call driven
by a queue reconfig request from an ethtool command or fw
recovery event.  We add a mutex around the rx_mode work to be
sure they don't collide.

Fixes: 81dbc241

 ("ionic: change set_rx_mode from_ndo to can_sleep")
Signed-off-by: Shannon Nelson <snelson@pensando.io>
Signed-off-by: David S. Miller <davem@davemloft.net>