ext4: fix use-after-free race with debug_want_extra_isize

ext4: fix use-after-free race with debug_want_extra_isize

commit 7bc04c5c2cc467c5b40f2b03ba08da174a0d5fa7 upstream.

When remounting with debug_want_extra_isize, we were not performing the
same checks that we do during a normal mount.  That allowed us to set a
value for s_want_extra_isize that reached outside the s_inode_size.

Fixes: e2b911c53584 ("ext4: clean up feature test macros with predicate functions")
Reported-by: syzbot+f584efa0ac7213c226b7@syzkaller.appspotmail.com
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Barret Rhoden <brho@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
[bwh: Backported to 4.9: The debug_want_extra_isize mount option is not
 supported]
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>