nfp: do not update MTU from BH in flower app

nfp: do not update MTU from BH in flower app

The Flower app may receive a request to update the MTU of a representor
netdev upon receipt of a control message from the firmware. This requires
the RTNL lock which needs to be taken outside of the packet processing
path.

As a handling of this correctly seems a little to invasive for a fix simply
skip setting the MTU for now.

Relevant backtrace:
 [ 1496.288489] BUG: scheduling while atomic: kworker/0:3/373/0x00000100
 [ 1496.294911]  dca syscopyarea sysfillrect sysimgblt fb_sys_fops ptp drm mxm_wmi ahci pps_core libahci i2c_algo_bit wmi [last unloaded: nfp]
 [ 1496.294918] CPU: 0 PID: 373 Comm: kworker/0:3 Tainted: G           OE   4.13.0-rc3+ #3
 [ 1496.294919] Hardware name: Supermicro X10DRi/X10DRi, BIOS 2.0 12/28/2015
 [ 1496.294923] Workqueue: events work_for_cpu_fn
 [ 1496.294924] Call Trace:
 [ 1496.294927]  <IRQ>
 [ 1496.294931]  dump_stack+0x63/0x82
 [ 1496.294935]  __schedule_bug+0x54/0x70
 [ 1496.294937]  __schedule+0x62f/0x890
 [ 1496.294941]  ? intel_unmap_sg+0x90/0x90
 [ 1496.294942]  schedule+0x36/0x80
 [ 1496.294943]  schedule_preempt_disabled+0xe/0x10
 [ 1496.294945]  __mutex_lock.isra.2+0x445/0x4a0
 [ 1496.294947]  ? device_is_rmrr_locked+0x12/0x50
 [ 1496.294950]  ? kfree+0x162/0x170
 [ 1496.294952]  ? device_is_rmrr_locked+0x12/0x50
 [ 1496.294953]  ? iommu_should_identity_map+0x50/0xe0
 [ 1496.294954]  __mutex_lock_slowpath+0x13/0x20
 [ 1496.294955]  ? iommu_no_mapping+0x48/0xd0
 [ 1496.294956]  ? __mutex_lock_slowpath+0x13/0x20
 [ 1496.294957]  mutex_lock+0x2f/0x40
 [ 1496.294960]  rtnl_lock+0x15/0x20
 [ 1496.294979]  nfp_flower_cmsg_rx+0xc8/0x150 [nfp]
 [ 1496.294986]  nfp_ctrl_poll+0x286/0x350 [nfp]
 [ 1496.294989]  tasklet_action+0xf6/0x110
 [ 1496.294992]  __do_softirq+0xed/0x278
 [ 1496.294993]  irq_exit+0xb6/0xc0
 [ 1496.294994]  do_IRQ+0x4f/0xd0
 [ 1496.294996]  common_interrupt+0x89/0x89

Fixes: 948faa46c05b ("nfp: add support for control messages for flower app")
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>