cxgbe(4): Stop and restart the L2T allocator with the LLD.

L2T entries are used by both filters and TOE and the L2T is shared
between the base driver (LLD) and the TOM ULD. Add a flag to indicate
that the L2T is stopped, which means:

• t4_alloc_l2e and t4_l2t_alloc_switching will not allocate new entries.
• t4_tom will ignore all ARP/NDP updates from the kernel.
• Previously allocated L2T entries can still be freed.

MFC after: 1 month
Sponsored by: Chelsio Communications