direct-io: defer alignment check until after the EOF check

Prior to commit 9fe55eea ("Fix race when checking i_size on direct
i/o read"), an unaligned direct read past end of file would trigger EOF,
since generic_file_aio_read detected this read-at-EOF condition and
skipped the direct IO read entirely, returning 0. After that change, the
read now reaches dio_generic, which detects the misalignment and returns
EINVAL.

This consolidates the generic direct-io to follow the same behavior of
filesystems.  Apparently, this fix will only affect ocfs2 since other
filesystems do this verification before calling do_blockdev_direct_IO,
with the exception of f2fs, which has the same bug, but is fixed in the
next patch.

it can be verified by a read loop on a file that does a partial read
before EOF (On file that doesn't end at an aligned address).  The
following code fails on an unaligned file on filesystems without
prior validation without this patch, but not on btrfs, ext4, and xfs.

  while (done < total) {
    ssize_t delta = pread(fd, buf + done, total - done, off + done);
    if (!delta)
      break;
    ...
  }

Fix this regression by moving the misalignment check to after the EOF
check added by commit 74cedf9b ("direct-io: Fix negative return from
dio read beyond eof").

Based on a patch by Jamie Liu.

Link: https://lore.kernel.org/r/20201008062620.2928326-4-krisman@collabora.com


Reported-by: Jamie Liu <jamieliu@google.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>
Signed-off-by: Jan Kara <jack@suse.cz>