ocfs2: Initialize the cluster we're writing to in a non-sparse extend

ocfs2: Initialize the cluster we're writing to in a non-sparse extend

commit e7432675f8ca868a4af365759a8d4c3779a3d922 upstream.

In a non-sparse extend, we correctly allocate (and zero) the clusters between
the old_i_size and pos, but we don't zero the portions of the cluster we're
writing to outside of pos<->len.

It handles clustersize > pagesize and blocksize < pagesize.

[Cleaned up by Joel Becker.]

Signed-off-by: Sunil Mushran <sunil.mushran@oracle.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>