coda_flag_children(): cope with dentries turning negative

->d_lock on parent does not stabilize ->d_inode of child.
We don't do much with that inode in there, but we need
at least to avoid struct inode getting freed under us...

[rcu_read_lock() is not needed here, since parent's ->d_lock
provides an rcu-critical area]

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>