mm/filemap.c: remove bogus VM_BUG_ON

mainline inclusion
from mainline-v5.16-rc1
commit d417b49f
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I6110W
cve: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d417b49fff3e2f21043c834841e8623a6098741d

--------------------------------

It is not safe to check page->index without holding the page lock.  It
can be changed if the page is moved between the swap cache and the page
cache for a shmem file, for example.  There is a VM_BUG_ON below which
checks page->index is correct after taking the page lock.

Link: https://lkml.kernel.org/r/20210818144932.940640-1-willy@infradead.org


Fixes: 5c211ba2 ("mm: add and use find_lock_entries")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reported-by:  <syzbot+c87be4f669d920c76330@syzkaller.appspotmail.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ma Wupeng <mawupeng1@huawei.com>
Reviewed-by: tong tiangen <tongtiangen@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>