cifs: Add comments for DeletePending assignments in open functions

On more places is set DeletePending member to 0. Add comments why is 0 the
correct value. Paths in DELETE_PENDING state cannot be opened by new calls.
So if the newly issued open for that path succeed then it means that the
path cannot be in DELETE_PENDING state.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>