The kernel can call these during unmount, so we have to handle them
directly to prevent any further IO being issued.
zfs_fsync reorganised slightly to not set up zfs_fsyncer_key until after
the teardown lock is acquired, just in case we don't get it.
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
(cherry picked from commit 900c26570ddcdd1d3ca135e6aee5df6456f6bfd6)