fs_mgr: zram: allow specifying stream count

This parameter allows specifying the number of parallel streams
to be used in compression and decompression. This option was added
in Linux 3.15, so it is not very common among devices currently on
the market, but it has been backported to some kernels. This option
must be set before zRAM is activated, as the kernel does not allow
switching from single stream to multistream compression once zRAM
is active.

Change-Id: I3f5ad96a65b3b4a6915d2700c398a236ea8931b0