ima: check return value of crypto_shash_final() in boot aggregate

The return value of crypto_shash_final() is not checked in
ima_calc_boot_aggregate_tfm(). If the hash finalization fails, the
function returns success and a corrupted boot aggregate digest could
be used for IMA measurements.

Capture the return value and propagate any error to the caller.

Fixes: 76bb28f6126f ("ima: use new crypto_shash API instead of old crypto_hash")
Signed-off-by: Daniel Hodges <hodgesd@meta.com>
Reviewed-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>