smb: client: correctly handle ErrorContextData as a flexible array

The `smb2_symlink_err_rsp` structure was previously defined with
`ErrorContextData` as a single `__u8` byte. However, the `ErrorContextData`
field is intended to be a variable-length array based on `ErrorDataLength`.
This mismatch leads to incorrect pointer arithmetic and potential memory
access issues when processing error contexts.

Updates the `ErrorContextData` field to be a flexible array
(`__u8 ErrorContextData[]`). Additionally, it modifies the corresponding
casts in the `symlink_data()` function to properly handle the flexible
array, ensuring correct memory calculations and data handling.

These changes improve the robustness of SMB2 symlink error processing.

Signed-off-by: Liang Jie <liangjie@lixiang.com>
Suggested-by: Tom Talpey <tom@talpey.com>
Signed-off-by: Steve French <stfrench@microsoft.com>