cifs: smb2ops: Fix NULL check in smb2_query_symlink

The current code null checks variable err_buf, which is always null
when it is checked, hence utf16_path is free'd and the function
returns -ENOENT everytime it is called, making it impossible for the
execution path to reach the following code:

err_buf = err_iov.iov_base;

Fix this by null checking err_iov.iov_base instead of err_buf. Also,
notice that err_buf no longer needs to be initialized to NULL.

Addresses-Coverity-ID: 1467876 ("Logically dead code")
Fixes: 2d636199e400 ("cifs: Change SMB2_open to return an iov for the error parameter")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Steve French <smfrench@gmail.com>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>