eth: fbnic: Add validation for MTU changes

eth: fbnic: Add validation for MTU changes

Increasing the MTU beyond the HDS threshold causes the hardware to
fragment packets across multiple buffers. If a single-buffer XDP program
is attached, the driver will drop all multi-frag frames. While we can't
prevent a remote sender from sending non-TCP packets larger than the MTU,
this will prevent users from inadvertently breaking new TCP streams.

Traditionally, drivers supported XDP with MTU less than 4Kb
(packet per page). Fbnic currently prevents attaching XDP when MTU is too high.
But it does not prevent increasing MTU after XDP is attached.

Fixes: 1b0a3950dbd4 ("eth: fbnic: Add XDP pass, drop, abort support")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Dimitri Daskalakis <dimitri.daskalakis1@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>