selinux: cleanup selinux_lsm_getattr()

A number of small changes to selinux_lsm_getattr() to improve the
quality and readability of the code:

* Explicitly set the `value` parameter to NULL in the case where an
  attribute has not been set.
* Rename the `__tsec` variable to `tsec` to better fit the SELinux code.
* Rename `bad` to `err_unlock` to better indicate the jump target drops
  the RCU lock.

Signed-off-by: Paul Moore <paul@paul-moore.com>