ext4: forbid i_extra_isize not divisible by 4

i_extra_isize not divisible by 4 is problematic for several reasons:

- It causes the in-inode xattr space to be misaligned, but the xattr
  header and entries are not declared __packed to express this
  possibility.  This may cause poor performance or incorrect code
  generation on some platforms.
- When validating the xattr entries we can read past the end of the
  inode if the size available for xattrs is not a multiple of 4.
- It allows the nonsensical i_extra_isize=1, which doesn't even leave
  enough room for i_extra_isize itself.

Therefore, update ext4_iget() to consider i_extra_isize not divisible by
4 to be an error, like the case where i_extra_isize is too large.

This also matches the rule recently added to e2fsck for determining
whether an inode has valid i_extra_isize.

This patch shouldn't have any noticeable effect on
non-corrupted/non-malicious filesystems, since the size of ext4_inode
has always been a multiple of 4.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>