zram: return error-valued pointer from zcomp_create()
Instead of returning just NULL, return ERR_PTR from zcomp_create() if
compressing backend creation has failed. ERR_PTR(-EINVAL) for unsupported
compression algorithm request, ERR_PTR(-ENOMEM) for allocation (zcomp or
compression stream) error.
Perform IS_ERR() check of returned from zcomp_create() value in
disksize_store() and set return code to PTR_ERR().