Upstream: unmount snapshots before destroying them on macOS

Add function zfs_destroy_snaps_nvl_os() call. The main issue is that
macOS needs to unmount any mounted snapshots before they can be
destroyed. Other platforms can handle this in the kernel, but sending
a storm of zed events to unmount seems undesirable when we can do it
in userland to start with.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Jorgen Lundman <lundman@lundman.net>
Co-authored-by: ilovezfs <ilovezfs@icloud.com>
Closes #12550