ext4: fix kernel infoleak via ext4_extent_header

Initialize eh_generation of struct ext4_extent_header to prevent leaking
info to userspace. Fixes KMSAN kernel-infoleak bug reported by syzbot at:
http://syzkaller.appspot.com/bug?id=78e9ad0e6952a3ca16e8234724b2fa92d041b9b8



Cc: stable@kernel.org
Reported-by:  <syzbot+2dcfeaf8cb49b05e8f1a@syzkaller.appspotmail.com>
Fixes: a86c6181 ("[PATCH] ext3: add extent map support")
Change-Id: I5f896c4e78a8c1a02a52c3466c6266bc37c8fdd2
Signed-off-by: Anirudh Rayabharam <mail@anirudhrb.com>
Link: https://lore.kernel.org/r/20210506185655.7118-1-mail@anirudhrb.com


Signed-off-by: Theodore Ts'o <tytso@mit.edu>
(cherry picked from commit ce3aba43)
Signed-off-by: Venkata Narasimha Bejawada <vbejawad@qti.qualcomm.com>