fuse: Do not mask return value from fuse_direct_io for partially valid data

fuse: Do not mask return value from fuse_direct_io for partially valid data

If a user calls writev/readv in direct io mode with partially valid data
in the iovec array such that any vector other than the first one in the
array contains invalid data, we currently return the error for the invalid
iovec.

Instead, we should return the number of bytes already written/read and not
the error as we do in the non direct_io case.

Orabug: 22732213

Reported-by: Alexey Kodanev <alexey.kodanev@oracle.com>
Signed-off-by: Ashish Samant <ashish.samant@oracle.com>
Acked-by: Miklos Szeredi <miklos@szeredi.hu>