staging: Add Snappy compression support to zram

Zram currently uses LZO compression. With Snappy, it uses less CPU time and is
thus more useful. The sacrifice in compression ratio is small.
Zram's LZO and Snappy support can be independently enabled at compile time and
each zram device can switch between compression methods when unused.
When only a single compression method was enabled at compile time, no idirection
penalty is incurred.

Signed-off-by: Zeev Tarantov <zeev.tarantov at gmail.com>
Signed-off-by: CTCaer <ctcaer@gmail.com>
Signed-off-by: garwedgess <garethwilliams21@gmail.com>