ima: prevent buffer overflow in ima_alloc_tfm()

This patch fixes the case where the file's signature/hash xattr contains
an invalid hash algorithm.  Although we can not verify the xattr, we still
need to measure the file.  Use the default IMA hash algorithm.

Signed-off-by: Dmitry Kasatkin <d.kasatkin@samsung.com>
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>