futex: Initialize futex_phash_new during fork().

During a hash resize operation the new private hash is stored in
mm_struct::futex_phash_new if the current hash can not be immediately
replaced.

The new hash must not be copied during fork() into the new task. Doing
so will lead to a double-free of the memory by the two tasks.

Initialize the mm_struct::futex_phash_new during fork().

Closes: https://lore.kernel.org/all/aFBQ8CBKmRzEqIfS@mozart.vkv.me/


Fixes: bd54df5e ("futex: Allow to resize the private local hash")
Reported-by: Calvin Owens <calvin@wbinvd.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Calvin Owens <calvin@wbinvd.org>
Link: https://lkml.kernel.org/r/20250623083408.jTiJiC6_@linutronix.de