ZTS: Fix faulty slog_replay_fs_001 test

ZTS: Fix faulty slog_replay_fs_001 test

This test is supposed to verify zil operations. For TX_WRITE, writes
must be synchronous in order to be entered in the zil. Linux seems to
be doing sync writes even when they are not asked for, but on FreeBSD
the test does not do what is intended.

Use dd oflag=sync for the parts of this test that are supposed to
result in TX_WRITE zil entries.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #10022