libcephfs: fix for #4068

If client->init() fails in mount, then client->shutdown()
should not be called. This patch uses a bool to ensure
that shutdown is only called if init() succeeds.

Signed-off-by: Joe Buck <jbbuck@gmail.com>
Reviewed-by: Sam Lang <sam.lang@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>