exec: move mac_smack_apply_pid() and setup_pam() to same condition block

exec: move mac_smack_apply_pid() and setup_pam() to same condition block

This cleans up exec_child() function by moving mac_smack_apply_pid()
and setup_pam() to the same condition block, since both of them have
the same condition (i.e params->apply_permissions). It improves
readability without changing its operation.