selftests: refactor the lsm `flags_overset_lsm_set_self_attr` test

Remove the temporary context variable `tctx` to simplify the code. use
the original context `ctx` directly in calls to `lsm_get_self_attr`,
eliminating redundancy without any functional changes.

Reviewed-by: Casey Schaufler <casey@schaufler-ca.com>
Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Amit Vadhavana <av2082000@gmail.com>
[PM: subject tweak]
Signed-off-by: Paul Moore <paul@paul-moore.com>