client: init dentry shared_gen with invalid value

client: init dentry shared_gen with invalid value

Directories and dentries are initialized with value 0 which makes detecting a
null  (or placeholder) dentry created via Client::get_or_create difficult.  We
already do checks to see if a dentry is invalid when the directory's shared_gen
changes so use an invalid value for these synthetic dentries until they can be
appropriately updated from Client::update_dentry_lease.

Signed-off-by: Patrick Donnelly <pdonnell@ibm.com>