Prevent duplicate mnttab cache entries

Under Linux its possible to mount the same filesystem multiple
times in the name space.  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.  A better solution may
be to use the full mount point as the AVL index rather than just
the file system name.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #2041