btrfs: return ptr error from compression workspace
The btrfs compression wrappers translated errors from workspace
allocation to either -ENOMEM or -1. The compression type workspace
allocators are already returning a ERR_PTR(-ENOMEM). Just return that
and get rid of the magical -1.
This helps a future patch return errors from the compression wrappers.
Signed-off-by: Zach Brown <zab@redhat.com> Reviewed-by: David Sterba <dsterba@suse.cz> Signed-off-by: Chris Mason <clm@fb.com>