image_types.bbclass: use 4096 instead of 8192 bytes-per-inode

The image not correctly created if 'ptest-pkgs' is in IMAGE_FEATURES,
this is because there is no free inode left. We can use 4096 instead of
8192 bytes-per-inode to fix the problem, and most of the distributions
us 4096, such as Ubuntu, Suse, Fedora and CentOS.

There are another problems:
* There are error message when there is no free inode left if we run the
  mke2fs command manually, but they are not in log.do_rootfs.

* The image generation doesn't stop when error happens because mke2fs
  doesn't return failed for this case.

Will fix them in other threads.

[YOCTO #5957]

(From OE-Core master rev: 09ab3a00598d06e3a1bf871811c2ac37359c74da)

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>