fs/fhandle.c: fix a race in call of has_locked_children()

In the Linux kernel, the following vulnerability has been resolved:

fs/fhandle.c: fix a race in call of has_locked_children()

may_decode_fh() is calling has_locked_children() while holding no locks. That's an oopsable race...

The rest of the callers are safe since they are holding namespace_sem and are guaranteed a positive refcount on the mount in question.

Rename the current has_locked_children() to __has_locked_children(), make it static and switch the fs/namespace.c users to it.

Make has_locked_children() a wrapper for __has_locked_children(), calling the latter under read_seqlock_excl(&mount_lock).