fuse: add simple request tracepoints

I've been timing various fuse operations and it's quite annoying to do
with kprobes.  Add two tracepoints for sending and ending fuse requests
to make it easier to debug and time various operations.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Bernd Schubert <bschubert@ddn.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>