ksmbd: fix error code overwriting in smb2_get_info_filesystem()

If client doesn't negotiate with SMB3.1.1 POSIX Extensions,
then proper error code won't be returned due to overwriting.

Return error immediately.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: e2f34481 ("cifsd: add server-side procedures for SMB3")
Cc: stable@vger.kernel.org
Signed-off-by: Matvey Kovalev <matvey.kovalev@ispras.ru>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>