ext4: zero out the unused memory region in the extent tree block

ext4: zero out the unused memory region in the extent tree block

CVE-2019-11833

This commit zeroes out the unused memory region in the buffer_head
corresponding to the extent metablock after writing the extent header
and the corresponding extent node entries.

This is done to prevent random uninitialized data from getting into
the filesystem when the extent block is synced.

This fixes CVE-2019-11833.

Signed-off-by: Sriram Rajagopalan <sriramr@arista.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org
(cherry picked from commit 592acbf16821288ecdc4192c47e3774a4c48bb64)
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>
Acked-by: Connor Kuehl <connor.kuehl@canonical.com>
Acked-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>