btrfs: constify input buffer parameter in compression code

btrfs: constify input buffer parameter in compression code

The input buffers passed down to compression must never be changed,
switch type to u8 as it's a raw byte buffer and use const.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>