decompress: Use 8 byte alignment

The ZSTD decompressor requires malloc() allocations to be 8 byte
aligned, so ensure that this the case.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20230807162720.545787-19-ardb@kernel.org