cifs: Fix duplicate fscache cookie warnings

fscache emits a lot of duplicate cookie warnings with cifs because the
index key for the fscache cookies does not include everything that the
cifs_find_inode() function does.  The latter is used with iget5_locked() to
distinguish between inodes in the local inode cache.

Fix this by adding the creation time and file type to the fscache cookie
key.

Additionally, add a couple of comments to note that if one is changed the
other must be also.

Signed-off-by: David Howells <dhowells@redhat.com>
Fixes: 70431bfd ("cifs: Support fscache indexing rewrite")
cc: Shyam Prasad N <nspmangalore@gmail.com>
cc: Rohith Surabattula <rohiths.msft@gmail.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: linux-cifs@vger.kernel.org
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Steve French <stfrench@microsoft.com>