smack: Always determine inode labels in smack_inode_init_security()

The inode_init_security hook is already a good place to initialize the
in-memory inode. And that is also what SELinux does.

In preparation for this, move the existing smack_inode_init_security() code
outside the 'if (xattr)' condition, and set the xattr, if provided.

This change does not have any impact on the current code, since every time
security_inode_init_security() is called, the initxattr() callback is
passed and, thus, xattr is non-NULL.

Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>