smack: Retrieve transmuting information in smack_inode_getsecurity()

commit 3a3d8fce31a49363cc31880dce5e3b0617c9c38b upstream.

Enhance smack_inode_getsecurity() to retrieve the value for
SMACK64TRANSMUTE from the inode security blob, similarly to SMACK64.

This helps to display accurate values in the situation where the security
labels come from mount options and not from xattrs.

Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
[4.19: adjusted for the lack of helper functions]
Fixes: d6d80cb5 ("Smack: Base support for overlayfs")
Signed-off-by: Munehisa Kamata <kamatam@amazon.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>