cifsd: add default case in switch statment in alloc_shash_desc()

Add default case in switch statment in alloc_shash_desc().

Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Steve French <stfrench@microsoft.com>