journaler: don't block when we adjust back write_pos
journaler: don't block when we adjust back write_pos

is_readable() may need to adjust the write_pos backward, but will return
false.  If we are at the end we still need to wake up any waiters so they
know about it.

Signed-off-by: Sage Weil <6dd34506bbd5e58221bbb3e4732d97c91f02277b@newdream.net>

ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph