LSM: Make some functions static

Fixes the following sparse warnings:

security/security.c:533:5: warning:
 symbol 'lsm_task_alloc' was not declared. Should it be static?
security/security.c:554:5: warning:
 symbol 'lsm_ipc_alloc' was not declared. Should it be static?
security/security.c:575:5: warning:
 symbol 'lsm_msg_msg_alloc' was not declared. Should it be static?

Fixes: f4ad8f2c ("LSM: Infrastructure management of the task security")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>
Signed-off-by: James Morris <james.morris@microsoft.com>