e2fsck: discard only unused parts of inode table

e2fsck: discard only unused parts of inode table

When calling e2fsck with '-E discard' option it might happen that
valid inodes are discarded accidentally. This is because we just
discard the part of inode table which lies past the free inode count.
This is terribly wrong (sorry!).

This patch fixes it so only the free parts of an inode table
is discarded, leaving used inodes intact. This was tested with highly
fragmented inode tables with block size 4k and 1k.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Reported-by: Phillip Susi <psusi@ubuntu.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>