selinux: avoid implicit conversions in the LSM hooks

Use the identical types in assignments of local variables for the
destination.

Merge tail calls into return statements.

Avoid using leading underscores for function local variable.

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
[PM: subject line tweaks]
Signed-off-by: Paul Moore <paul@paul-moore.com>