fix do_emergency_remount_callback() and do_umount_root()

The former is racy - fetching ->s_root outside of ->s_umount.
The latter actually never propagates errors (ret in inner
scope shadows ret in the outer, so any assignments are lost
by the time we get to return ret;).

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>