Miscellaneous futex fixes:

 - Series to enforce that private futex owner shares the mm when
   attaching. (Kyle Zeng, Thomas Gleixner)

 - Fix race on the initial mm->futex.phash.ref allocation
   (Hyunwoo Kim)

 - Fix might_sleep() warning in futex_pivot_pending()
   (Peter Zijlstra)

Signed-off-by: Ingo Molnar <mingo@kernel.org>

futex: Fix might_sleep() warning in futex_pivot_pending()

A younger me put a WARN in might_sleep() to warn about nested sleep loops. This
younger me also build a wait-loop variant that can deal with it. This wait-loop
variant doesn't have all the fancy wrappers, since it isn't used much. It also
lacks wait-bit support.

Add the wait-bit support and use it to fix the nested wait issue.

Fixes: 8e7ff730dd96 ("futex: Fix race in futex_pivot_pending() during private hash resize")
Reported-by: syzbot+350a93852ac854927f45@syzkaller.appspotmail.com
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Link: https://patch.msgid.link/20260820074927.GH1246887@noisy.programming.kicks-ass.net
Closes: https://syzkaller.appspot.com/bug?extid=350a93852ac854927f45