vfs: show_vfsstat: do not ignore errors from show_devname method

Explicitly check show_devname method return code and bail out in case
of an error.  This fixes regression introduced by commit 9d4d6574.

Cc: stable@vger.kernel.org
Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>