smack: remove redundant structure variable from header.

commit afb1cbe3

 ("LSM: Infrastructure management
of the inode security") removed usage of smk_rcu,
thus removing it from structure.

Signed-off-by: Maninder Singh <maninder1.s@samsung.com>
Signed-off-by: Vaneet Narang <v.narang@samsung.com>
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>