ext4: add more paranoia checking in ext4_expand_extra_isize handling

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>
Cc: stable@kernel.org