initramfs: use mount.zfs instead of mount

A followup to d7a6740

For `mount -t zfs -o opts ds mp` command line
some implementations of `mount(8)`, e. g. Busybox in Debian
work as follows:

```
newfstatat(AT_FDCWD, "ds", 0x7fff826f4ab0, 0) = -1
mount("ds", "mp", "zfs", MS_SILENT, NULL) = 0
```

The logic above skips completely `mount.zfs` and prevents us
from reading filesystem properties and applying mount options.

For comparison, the coreutils `mount(8)` implementation does:

```
openat(AT_FDCWD, "/proc/filesystems", O_RDONLY|O_CLOEXEC) = 3
// figure out that zfs is a `nodev` filesystem and look for a helper
newfstatat(AT_FDCWD, "/sbin/mount.zfs" ...) = 0
execve("/sbin/mount.zfs" ...) = 0
```

Using `mount.zfs` in initramfs would help circumvent deficiencies
of some of `mount(8)` implementations. `mount -t zfs` translates
to `mount.zfs` invocation, except for cases when explicitly disabled
by `-i`.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: szubersk <szuberskidamian@gmail.com>
Closes #13305

contrib/initramfs/scripts/zfs

@@ -328,7 +328,7 @@ mount_fs()
 
 	# Need the _original_ datasets mountpoint!
 	mountpoint=$(get_fs_value "$fs" mountpoint)
-	ZFS_CMD="mount -o zfsutil -t zfs"
+	ZFS_CMD="mount.zfs -o zfsutil"
 	if [ "$mountpoint" = "legacy" ] || [ "$mountpoint" = "none" ]; then
 		# Can't use the mountpoint property. Might be one of our
 		# clones. Check the 'org.zol:mountpoint' property set in
@@ -351,7 +351,7 @@ mount_fs()
 		# If it's not a legacy filesystem, it can only be a
 		# native one...
 		if [ "$mountpoint" = "legacy" ]; then
-			ZFS_CMD="mount -t zfs"
+			ZFS_CMD="mount.zfs"
 		fi
 	fi
 
@@ -919,7 +919,7 @@ mountroot()
 		echo "       not specified on the kernel command line."
 		echo ""
 		echo "Manually mount the root filesystem on $rootmnt and then exit."
-		echo "Hint: Try:  mount -o zfsutil -t zfs ${ZFS_RPOOL-rpool}/ROOT/system $rootmnt"
+		echo "Hint: Try:  mount.zfs -o zfsutil ${ZFS_RPOOL-rpool}/ROOT/system $rootmnt"
 		shell
 	fi