zvol: Fix blk-mq sync

The zvol blk-mq codepaths would erroneously send FLUSH and TRIM
commands down the read codepath, rather than write. This fixes
the issue, and updates the zvol_misc_fua test to verify that
sync writes are actually happening.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Reviewed-by: Ameer Hamza <ahamza@ixsystems.com>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #17761
Closes #17765