mm/mempolicy: fix memory leaks in weighted_interleave_auto_store()

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

mm/mempolicy: fix memory leaks in weighted_interleave_auto_store()

weighted_interleave_auto_store() fetches old_wi_state inside the if
(!input) block only. This causes two memory leaks:

1. When a user writes "false" and the current mode is already manual,
the function returns early without freeing the freshly allocated
new_wi_state.

2. When a user writes "true", old_wi_state stays NULL because the
fetch is skipped entirely. The old state is then overwritten by
rcu_assign_pointer() but never freed, since the cleanup path is
gated on old_wi_state being non-NULL. A user can trigger this
repeatedly by writing "1" in a loop.

Fix both leaks by moving the old_wi_state fetch before the input check,
making it unconditional. This also allows a unified early return for both
"true" and "false" when the requested mode matches the current mode.

Reviewed by: Donet Tom <donettom@linux.ibm.com>