cifs: convert to use ERR_CAST()

Use ERR_CAST() as it is designed for casting an error pointer to
another type.

This macro uses the __force and __must_check modifiers, which are used
to tell the compiler to check for errors where this macro is used.

Signed-off-by: Yuesong Li <liyuesong@vivo.com>
Signed-off-by: Steve French <stfrench@microsoft.com>