Fix: update btrfs instrumentation for kernel 4.14

Fix: update btrfs instrumentation for kernel 4.14

See upstream commit:

  Author: Jeff Mahoney <jeffm@suse.com>
  Date:   Wed Jun 28 21:56:54 2017 -0600

    btrfs: constify tracepoint arguments

    Tracepoint arguments are all read-only.  If we mark the arguments
    as const, we're able to keep or convert those arguments to const
    where appropriate.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>