cifs: small underflow in cnvrtDosUnixTm()

[ Upstream commit 564277eceeca01e02b1ef3e141cfb939184601b4 ]

January is month 1.  There is no zero-th month.  If someone passes a
zero month then it means we read from one space before the start of the
total_days_of_prev_months[] array.

We may as well also be strict about days as well.

Fixes: 1bd5bbcb ("[CIFS] Legacy time handling for Win9x and OS/2 part 1")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Steve French <smfrench@gmail.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>