rt-table: Fix kernel protocol export filter memory bug

Kernel protocol calls rt_export_merged(), which used @rte_update_pool for
temporary allocations, supposing it is called from other functions from
rt-table.c that handles locking and flushing of the linpool. Therefore,
linpool was not flushed properly and memory leaked.

Add linpool argument to rt_export_merged() and use @krt_filter_lp when
called from kernel protocol.

Thanks to Justin Cattle and Alexander Frolkin for the bugreport.

(Commit squashed and updated by Ondrej Zajicek)