dtrace probes

Hi,

The lwlock dtrace probes define LWLockMode as int, and the
TRACE_POSTGRESQL_LWLOCK methods are called using both a variable and
constant definition.

This leads to a mix of argument definitions depending on the call site,
as seen in probes.txt file.

A fix is to explicit cast 'mode' to int such that all call sites will
use the

argument #2 4 signed bytes

definition. Attached patch does this.

I have verified all dtraces probes for their type, and only the lock__
methods doesn't aligned with its actual types. However, that would
require a change to probes.d, and therefore PostgreSQL 11 material.

Depending on the feedback I can add this patch to the open item list in
order to fix it for PostgreSQL 10.

Best regards,
Jesper