LU-15184 llite: properly detect SELinux disabled case

LU-15184 llite: properly detect SELinux disabled case

Usually, security_dentry_init_security() returns -EOPNOTSUPP when
SELinux is disabled. But on some kernels (e.g. rhel 8.5) it returns
0 when SELinux is disabled, and in this case the security context is
empty.
So in both cases make sure the security context name is not set, which
means "SELinux is disabled" for the rest of the code.

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I3b9608f9768288de89570c158e8429560fa0213f