apparmor: Fix -Wdiscarded-qualifiers warning in deflate_compress

The no-compression case was taking a shortcut in deflate_compress() which
just returned a pointer to the source buffer, but this triggered a
-Wdiscarded-qualifiers warning because the source buffer argument is
declared as const. Making the source buffer argument in the
deflate_compress() signature non-const feels strange, so move the
no-compression shortcut up to compress_loaddata() instead.