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

Vulnerability Description

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).

References

• https://git.kernel.org/stable/c/6482c3dccbfb8d20e2856ce67c75856859930b3f
• https://git.kernel.org/stable/c/287c7d34eedd37af1272dfb3b6e8656f4f026424
• https://git.kernel.org/stable/c/1f282cdc1d219c4a557f7009e81bc792820d9d9a