selinux: remove extraneous initialization of slots_used and max_chain_len

Variables slots_used and max_chain_len are being initialized to zero
twice. Remove the second set of initializations in the for loop.
Cleans up the clang warnings:

Value stored to 'slots_used' is never read
Value stored to 'max_chain_len' is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>