cls/rbd: init local var with known value

cls/rbd: init local var with known value

DirectoryState does not have an "invalid" enum so far, since it's
defined using `enum class`, init a value of this type with a known value
could be a better choice even it is always initialized before being read.

this silences the GCC warning of:

src/cls/rbd/cls_rbd.cc:3147:3: warning: ‘on_disk_directory_state’ may be
used uninitialized in this function [-Wmaybe-uninitialized]
   if (directory_state != on_disk_directory_state) {
   ^~

Signed-off-by: Kefu Chai <kchai@redhat.com>