ext4: fix use-after-free in dx_release()

ext4: fix use-after-free in dx_release()

BugLink: https://bugs.launchpad.net/bugs/1836614
commit 08fc98a4d6424af66eb3ac4e2cedd2fc927ed436 upstream.

The buffer_head (frames[0].bh) and it's corresping page can be
potentially free'd once brelse() is done inside the for loop
but before the for loop exits in dx_release(). It can be free'd
in another context, when the page cache is flushed via
drop_caches_sysctl_handler(). This results into below data abort
when accessing info->indirect_levels in dx_release().

Unable to handle kernel paging request at virtual address ffffffc17ac3e01e
Call trace:
 dx_release+0x70/0x90
 ext4_htree_fill_tree+0x2d4/0x300
 ext4_readdir+0x244/0x6f8
 iterate_dir+0xbc/0x160
 SyS_getdents64+0x94/0x174

Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Cc: stable@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>