evm: prohibit userspace writing 'security.evm' HMAC value

commit 2fb1c9a4 upstream.

Calculating the 'security.evm' HMAC value requires access to the
EVM encrypted key.  Only the kernel should have access to it.  This
patch prevents userspace tools(eg. setfattr, cp --preserve=xattr)
from setting/modifying the 'security.evm' HMAC value directly.

Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>