futex: Fix inode life-time issue

commit 8019ad13 upstream.

As reported by Jann, ihold() does not in fact guarantee inode
persistence. And instead of making it so, replace the usage of inode
pointers with a per boot, machine wide, unique inode identifier.

This sequence number is global, but shared (file backed) futexes are
rare enough that this should not become a performance issue.

Reported-by: Jann Horn <jannh@google.com>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Change-Id: If2df98716d113c6cd6922d8e103f1a4adf6c994a
Git-commit: e6d506cd
Git-repo: https://android.googlesource.com/kernel/msm


Signed-off-by: PavanKumar S.R. <pavasr@codeaurora.org>