libext2fs: fix uninit block calculation when inodes_per_block < first_ino

libext2fs: fix uninit block calculation when inodes_per_block < first_ino

The following commands:

	dd if=/dev/zero of=/tmp/foo count=1 ibs=$(( 256 * 1024 * 1024 ))
	mke2fs -N 256 -t ext4 /tmp/foo

... will cause mke2fs to write until it fills the device.  The cause
for this is that the explicit request for 256 inodes causes the number
of inodes per block group to be 8.  The ext2fs_initialize() function
assumed that all of the reserved inodes would be in the first block
group, which is not true in this case.  This caused the number of
uninitialized inodes in the first block group to be negative, which
then resulted in mke2fs trying to zero out a very large number of
blocks.  Oops.

Addresses-Sourceforge-Bug: #3528892

Reported-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>