librbd: fix ictx_check pointer weirdness by using std::string

librbd: fix ictx_check pointer weirdness by using std::string

I was seeing failures of LibRBD.TestIOToSnapshot where we would fail to
refresh after rollback, even though the snap existed.  I assume it is
because the std::string whose c_str() we were pointing to was reallocated.

Use a std::string here instead.

This code is weird.

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