zram-swap: Add "max compression streams" configuration option

Config option to limit maximum compression streams per zram dev for
multicore CPU's. This could be defined via 'zram_comp_streams' option in
the 'system' section of '/etc/config/system' file or via cli (for e.x.
with 'uci set system.@System[0].zram_comp_streams=2 && uci commit
system'). Default is number of logical CPU cores.

Signed-off-by: Emil Muratov <gpm@hotplug.ru>
(backported from 70d3ffb47fcef901e4d86da4c9077ba8b8e2ba10)