Prevent duplicate mnttab cache entries

Under Linux its possible to mount the same filesystem multiple
times in the namespace. This can be done either with bind mounts
or simply with multiple mount points. Unfortunately, the mnttab
cache code is implemented using an AVL tree which does not support
duplicate entries. To avoid this issue this patch updates the
code to check for a duplicate entry before adding a new one.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Michael Martin <mgmartin.mgm@gmail.com>
Closes #2041