ext4: add sysfs attr /sys/fs/ext4/<disk>/journal_task

This is useful for moving journal thread into cgroup or
for tracing it with ftrace/perf/blktrace.

For now the only way is `pgrep jbd2/$DISK` but this is not reliable:
name may be longer than "comm" limit and any task could mock it.

Attribute shows pid in current pid-namespace or 0 if task is unreachable.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>