cifs: pass a path to open_shroot and check if it is the root or not

Move the check for the directory path into the open_shroot() function
but still fail for any non-root directories.
This is preparation for later when we will start using the cache also
for other directories than the root.

Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>