KEYS: trusted: dcp: fix improper sg use with CONFIG_VMAP_STACK=y

In the Linux kernel, the following vulnerability has been resolved: KEYS: trusted: dcp: fix improper sg use with CONFIG_VMAP_STACK=y With vmalloc stack addresses enabled (CONFIG_VMAP_STACK=y) DCP trusted keys can crash during en- and decryption of the blob encryption key via the DCP crypto driver. This is caused by improperly using sg_init_one() with vmalloc'd stack buffers (plain_key_blob). Fix this by always using kmalloc() for buffers we give to the DCP crypto driver.