generic/465: just check the actual read data under dio read/write

generic/465: just check the actual read data under dio read/write

I got the following message when running generic/465 in ext4
data=journal mode
---------------------------------------------------------------
QA output created by 465
non-aio dio test
encounter an error: block 0 offset 4096, content 62
encounter an error: block 0 offset 122880, content 62
encounter an error: block 0 offset 274432, content 62
encounter an error: block 0 offset 86016, content 62
aio-dio test
encounter an error: block 0 offset 28672, content 62
encounter an error: block 0 offset 12288, content 62
encounter an error: block 2 offset 16384, content 62
encounter an error: block 1 offset 565248, content 62
---------------------------------------------------------------

In ext4 data=journal mode, direct read will fall back to buffer
read, and buffer read doesn't take inode lock, so it doesn't need to
wait for the writer to finish first and sees the intermediate inode
size and returns data less than 1M.

We can just check the actual read data instead of the whole read
buffer.

Signed-off-by: Xiao Yang <yangx.jy@cn.fujitsu.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>