writeback: use ino_t for inodes in tracepoints

Writeback TPs currently use mix of 32 and 64bits for inos.  This isn't
currently broken because only cgroup inos are using 32bits and they're
limited to 32bits.  cgroup inos will make use of 64bits.  Let's
uniformly use ino_t.

While at it, switch the default cgroup ino value used when cgroup is
disabled to 1 instead of -1U as root cgroup always uses ino 1.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Namhyung Kim <namhyung@kernel.org>