don't use O_DIRECT in xfsdump for RT files

[Christoph Hellwig <hch@xxxxxx>]

Hi all,

for reasons unknown to me xfsdump has been using O_DIRECT for files on
the RT subvolume since day 1.  But on 4k file system this causes writes
to fail due to alignment problem, which shows up in xfstests xfs/060.

This removes the O_DIRET flag and treats RT files like files on the
data device and thus fixes the test failure.

(xfs/059 still fails on 4k file system when using rthinherit with what
looks file data corruption, I'll look into that next)