initramfs: Honor canmount=off

The initramfs script was not honoring canmount=off. With this change,
it does. If the administrator has asked that a filesystem not be
mounted, that should be honored.

As an exception, the initramfs script ignores canmount=off on the
rootfs. The rootfs should not have canmount=off set either. However,
mounting it anyway seems harmless because it is being asked for
explicitly. The point of this exception is to avoid the risk of
breaking existing systems, just in case someone has canmount=off set on
their rootfs.

The initramfs still mounts filesystems with canmount=noauto. This is
necessary because it is typical to set that on the rootfs so that it can
be cloned. Without canmount=noauto, the clones' duplicate mountpoints
would conflict.

This is the remainder of the fix for:
https://github.com/zfsonlinux/pkg-zfs/issues/221

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Richard Laager <rlaager@wiktel.com>
Closes #6897