Revert "Let zfs mount all tolerate in-progress mounts"

This reverts commit a9cd8bf which introduced a segfault when running
zfs mount -a multiple times when there are mountpoints which are
not empty. This segfault is now seen frequently by the CI after
the mount code was updated to directly call mount(2).

The original reason this logic was added is described in #8881.
Since then the systemd zfs-share.target has been updated to run
"After" the zfs-mount.server which should avoid this issue.

Reviewed-by: Don Brady <don.brady@delphix.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #9560
Closes #10364