net/mlx4_en: Issue the dump eth statistics command under lock

Performing the DUMP_ETH_STATS firmware command outside the lock leads to kernel
panic when data structures such as RX/TX rings are freed in parallel, e.g when
one changes the mtu or ring sizes.

Signed-off-by: Eugenia Emantayev <eugenia@mellanox.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>