mount.zfs: canonicalize mount point for mtab

Canonicalize the mount point passed to the mount.zfs helper.
This way a clean path is always added to mtab which ensures
the umount can properly locate and remove the entry.

Test case:
$ mkdir /mnt/foo
$ mount -t zfs zpool/foo /mnt/../mnt/foo//
$ umount /mnt/foo
$ cat /etc/mtab | grep zpool/foo
zpool/foo /mnt/../mnt/foo// zfs rw 0 0

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #573