cifs: Grab a reference for the dentry of the cached directory during the lifetime of the cache

We need to hold both a reference for the root/superblock as well as the directory that we
are caching. We need to drop these references before we call kill_anon_sb().

At this point, the root and the cached dentries are always the same but this will change
once we start caching other directories as well.

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