e2fsck: do not discard itable if discard doen't zero data

e2fsck: do not discard itable if discard doen't zero data

We do not want to discard inode table if the underlying device does not
return zeros when reading non-provisioned blocks. The reason is that if
the inode table is not zeroed yet, then discard would not help us since
we would have to zero it anyway. In the case that inode table was
already zeroed, then the discard would cause subsequent reads to contain
non-deterministic data so we would not be able to assume that the inode
table was zeroed and we would need to zero it again, which does not
really make sense.

This commit adds check to prevent inode table from being discarded if
the discard does not zero data.

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