fsync-tester: reopen files with correct flags

fsync-tester: reopen files with correct flags

When "drop caches" tests are run, the test_fd is closed and
re-opened. It is reopened O_RDWR, so dropping O_DIRECT and any other
open flag that was set for test_fd. This fixes a random generic/311
failure due to file data corruption I've been seeing on subtests 11
and 13 when direct IO was specified.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>