Smack: Mark inode instant in smack_task_to_inode

/proc clean-up in commit 1bbc5513
resulted in smack_task_to_inode() being called before smack_d_instantiate.
This resulted in the smk_inode value being ignored, even while present
for files in /proc/self. Marking the inode as instant here fixes that.
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
Signed-off-by: James Morris <james.morris@microsoft.com>