xfs: correct the truncate blocksize of forcealign

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I9VTE3


CVE: NA

--------------------------------

Now xfs_itruncate_extents() unmap extsize aligned extents for forcealign,
when unaligned truncating down a forceflign file which extsize is bigger
than one block, xfs_truncate_page() only zeros out the tail EOF block,
this could expose stale data.

If we truncate file that contains a large enough written extent:

     |<      ext    >|<      ext    >|
  ...WWWWWWWWWWWWWWWWWWWWWzzzzzzzzzzzz
        ^ (new EOF)      ^ old EOF

Since we only zeros out the tail of the EOF block, and
xfs_itruncate_extents() unmap the whole ailgned extents, it becomes
this state:

     |<      ext    >|
  ...WWWzWWWWWWWWWWWWW
        ^ new EOF

Then if we do an extending write like this, the blocks in the previous
tail extent becomes stale:

     |<      ext    >|      |<     ext     >|
  ...WWWzSSSSSSSSSSSSS......WWWWWWWWWWWzzzzzz
        ^ old EOF     ^ append start  ^ new EOF

Fix this by zeroing out the tail allocation uint for forcealign.

Fixes: fabcdd2d ("fs: xfs: Introduce FORCEALIGN inode flag")
Signed-off-by: Long Li <leo.lilong@huawei.com>