Smack: Improve mount process memory use

The existing mount processing code in Smack makes many unnecessary
copies of Smack labels. Because Smack labels never go away once
imported it is safe to use pointers to them rather than copies.
Replace the use of copies of label names to pointers to the global
label list entries.

Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>