evm: Execute evm_inode_init_security() only when an HMAC key is loaded

evm_inode_init_security() requires an HMAC key to calculate the HMAC on
initial xattrs provided by LSMs. However, it checks generically whether a
key has been loaded, including also public keys, which is not correct as
public keys are not suitable to calculate the HMAC.

Originally, support for signature verification was introduced to verify a
possibly immutable initial ram disk, when no new files are created, and to
switch to HMAC for the root filesystem. By that time, an HMAC key should
have been loaded and usable to calculate HMACs for new files.

More recently support for requiring an HMAC key was removed from the
kernel, so that signature verification can be used alone. Since this is a
legitimate use case, evm_inode_init_security() should not return an error
when no HMAC key has been loaded.

This patch fixes this problem by replacing the evm_key_loaded() check with
a check of the EVM_INIT_HMAC flag in evm_initialized.

Fixes: 26ddabfe ("evm: enable EVM when X509 certificate is loaded")
Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Cc: stable@vger.kernel.org # 4.5.x
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>