Revert "mke2fs: prevent creation of unmountable ext4 with large flex_bg count"

Revert "mke2fs: prevent creation of unmountable ext4 with large flex_bg count"

This reverts commit d988201ef9cb6f7b521e544061976ab4270a3f89.

The problem with this commit is that causes common small file system
configurations to fail.  For example:

    mke2fs -O flex_bg -b 4096 -I 1024 -F /tmp/tt 79106
    mke2fs 1.42.11 (09-Jul-2014)
    /tmp/tt: Invalid argument passed to ext2 library while setting
             up superblock

This check in ext2fs_initialize() was added to prevent the metadata
from being allocated beyond the end of the filesystem, but it is
also causing a wide range of failures for small filesystems.

We'll address this in a different way, by using a smarter algorithm
for deciding the layout of metadata blocks for the last flex block
group.

Reported-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>