ima: Fix memory leak in __ima_inode_hash()

Commit f3cc6b25 ("ima: always measure and audit files in policy") lets
measurement or audit happen even if the file digest cannot be calculated.

As a result, iint->ima_hash could have been allocated despite
ima_collect_measurement() returning an error.

Since ima_hash belongs to a temporary inode metadata structure, declared
at the beginning of __ima_inode_hash(), just add a kfree() call if
ima_collect_measurement() returns an error different from -ENOMEM (in that
case, ima_hash should not have been allocated).

Cc: stable@vger.kernel.org
Fixes: 280fe836

 ("ima: Always return a file measurement in ima_file_hash()")
Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>