btrfs: merge length input and output parameter in compress_pages

btrfs: merge length input and output parameter in compress_pages

The length parameter is basically duplicated for input and output in the
top level caller of the compress_pages chain. We can simply use one
variable for that and reduce stack consumption. The compression
implementation will sink the parameter to a local variable so everything
works as before.

Signed-off-by: David Sterba <dsterba@suse.com>