ext2/xip: refuse to change xip flag during remount with busy inodes

For a reason that I was unable to understand in three months of debugging,
mount ext2 -o remount stopped working properly when remounting from
regular operation to xip, or the other way around. According to a git
bisect search, the problem was introduced with the VM_MIXEDMAP/PTE_SPECIAL
rework in the vm:

commit 70688e4dd1647f0ceb502bbd5964fa344c5eb411
Author: Nick Piggin <npiggin@suse.de>
Date: Mon Apr 28 02:13:02 2008 -0700

xip: support non-struct page backed memory

In the failing scenario, the filesystem is mounted read only via root=
kernel parameter on s390x. During remount (in rc.sysinit), the inodes of
the bash binary and its libraries are busy and cannot be invalidated (the
bash which is running rc.sysinit resides on subject filesystem).
Afterwards, another bash process (running ifup-eth) recurses into a
subshell, runs dup_mm (via fork). Some of the mappings in this bash
process were created from inodes that could not be invalidated during
remount.

Both parent and child process crash some time later due to inconsistencies
in their address spaces. The issue seems to be timing sensitive, various
attempts to recreate it have failed.

This patch refuses to change the xip flag during remount in case some
inodes cannot be invalidated. This patch keeps users from running into
that issue.

[akpm@linux-foundation.org: cleanup]
Signed-off-by: Carsten Otte <cotte@de.ibm.com>
Cc: Nick Piggin <npiggin@suse.de>
Cc: Jared Hulbert <jaredeh@gmail.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>