libceph: lower exponential backoff delay

The current setting allows the backoff to climb up to 5 minutes.  This
is too high -- it becomes hard to tell whether the client is stuck on
something or just in backoff.

In userspace, ms_max_backoff is defaulted to 15 seconds.  Let's do the
same.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>