Various fixes to the mlxen(4) driver:
- Remove an incorrect assertion that can trigger when downing an interface.
- Stop the interface during detach to avoid panics when unloading the
  driver.
- A few locking fixes to be more consistent with other FreeBSD drivers:
  - Protect if_drv_flags with the driver lock, not atomic ops
  - Hold the driver lock when adjusting multicast state.
  - Hold the driver lock while adjusting if_capenable.

PR:		kern/180791 [1,2]
Submitted by:	Shakar Klein @ Mellanox [1,2]
MFC after:	3 days
Unbreak sysctl ABI changes introduced in r253662

Requested by:	bde