IMA: block writes of the security.ima xattr with unsupported algorithms

By default, writes to the extended attributes security.ima will be
allowed even if the hash algorithm used for the xattr is not compiled
in the kernel (which does not make sense because the kernel would not
be able to appraise that file as it lacks support for validating the
hash).

Prevent and audit writes to the security.ima xattr if the hash algorithm
used in the new value is not available in the current kernel.

Signed-off-by: THOBY Simon <Simon.THOBY@viveris.fr>
Reviewed-by: Lakshmi Ramasubramanian <nramas@linux.microsoft.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>