generic: clear of suid/sgid on direct write

generic: clear of suid/sgid on direct write

Check that suid/sgid bits are cleared on direct write. XFS triggered
WARN_ON_ONCE in this case. Patchset from Jan Kara fixed the warning:

http://oss.sgi.com/archives/xfs/2014-12/msg00071.html

This test is inspired by a test case from Eric Sandeen, and follows
the test steps in generic/193. This test requires direct I/O, it's
not added to generic/193 but to a new test, so that generic/193
still runs on filesystems don't have direct I/O support.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eryu Guan <eguan@redhat.com>