selinux: access superblock_security_struct in LSM blob way

LSM blob has been involved for superblock's security struct. So fix the
remaining direct access to sb->s_security by using the LSM blob
mechanism.

Fixes: 08abe46b ("selinux: fall back to SECURITY_FS_USE_GENFS if no xattr support")
Fixes: 69c4a42d

 ("lsm,selinux: add new hook to compare new mount to an existing mount")
Signed-off-by: GONG, Ruiqi <gongruiqi1@huawei.com>
Reviewed-by: Casey Schaufler <casey@schaufler-ca.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>