ext4: add more paranoia checking in ext4_expand_extra_isize handling

commit 4ea99936 upstream.

It's possible to specify a non-zero s_want_extra_isize via debugging
option, and this can cause bad things(tm) to happen when using a file
system with an inode size of 128 bytes.

Add better checking when the file system is mounted, as well as when
we are actually doing the trying to do the inode expansion.

Link: https://lore.kernel.org/r/20191110121510.GH23325@mit.edu


Reported-by:  <syzbot+f8d6f8386ceacdbfff57@syzkaller.appspotmail.com>
Reported-by:  <syzbot+33d7ea72e47de3bdf4e1@syzkaller.appspotmail.com>
Reported-by:  <syzbot+44b6763edfc17144296f@syzkaller.appspotmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
[bwh: Backported to 3.16:
 - Use EIO instead of EFSCORRUPTED
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>