ext4: add bounds checking in get_max_inline_xattr_value_size()

Normally the extended attributes in the inode body would have been
checked when the inode is first opened, but if someone is writing to
the block device while the file system is mounted, it's possible for
the inode table to get corrupted.  Add bounds checking to avoid
reading beyond the end of allocated memory if this happens.

Reported-by:  <syzbot+1966db24521e5f6e23f7@syzkaller.appspotmail.com>
Link: https://syzkaller.appspot.com/bug?extid=1966db24521e5f6e23f7


Cc: stable@kernel.org
Signed-off-by: Theodore Ts'o <tytso@mit.edu>