libext2fs: avoid buffer overflow if s_first_meta_bg is too big

If s_first_meta_bg is greater than the of number block group
descriptor blocks, then reading or writing the block group descriptors
will end up overruning the memory buffer allocated for the
descriptors.  Fix this by limiting first_meta_bg to no more than
fs->desc_blocks.  This doesn't correct the bad s_first_meta_bg value,
but it avoids causing the e2fsprogs userspace programs from
potentially crashing.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
e2fsck: fix file systems with an overly large s_first_meta_bg

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
resize2fs: disable the meta_bg feature if necessary

When shrinking a file system, if the number block groups drops below
the point where we started using the meta_bg layout, disable the
meta_bg feature and set s_first_meta_bg to zero.  This is necessary to
avoid creating an invalid/corrupted file system after the shrink.

Addresses-Debian-Bug: #756922

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reported-by: Marcin Wolcendorf <antymat+debian@chelmska.waw.pl>
Tested-by: Marcin Wolcendorf <antymat+debian@chelmska.waw.pl>
tests: make sure MKE2FS_FIRST_META_BG is unset while running tests

If the developer has set the MKE2FS_FIRST_META_BG environment
variable, this can cause test failures.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
tests: add f_first_meta_bg_too_big test

The test verifies that e2fsck can properly fix a file system where the
value of s_first_meta_bg in the superblock is larger than the number
of block group descriptors in the file system.  E2fsck will fix this
by clearing the meta_bg feature.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
tests: add the r_meta_bg_shrink test

This test checks to make sure resize2fs can properly handle a file
system which started life as a normal ext4 file system and then was
grown to a size where meta_bg was enabled, and then shrunk back below
the point where the meta_bg format is still needed.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
e2fsck: flush out the superblock and bitmaps before printing final messages

A user who sees the message

***** REBOOT LINUX *****

or

***** FILE SYSTEM WAS MODIFIED *****

might think that e2fsck was complete even though we haven't finished
writing out the superblock or bitmap blocks, and then either forcibly
reboot or power cycle the box, or yank the USB key out while the
storage device is still being written (before e2fsck exits).

So rearrange the exit path of e2fsck so that we flush out the dirty
superblock/bg descriptors/bitmaps before we print the final message.
Also clean up this code so that the flow of control is easier to
understand, and add error checking to catch any errors (normally
caused by I/O errors writing to the disk) for these final writebacks.

Addresses-Debian-Bugs: #757543, #757544
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: Dan Jacobson <jidanni@jidanni.org>