ksmbd: fix memory leak smb2_populate_readdir_entry()

Add missing kfree(conv_name) on error path.

Reported-by: Coverity Scan <scan-admin@coverity.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Steve French <stfrench@microsoft.com>