Fix zfs_unmount() and zfs_unshare_proto() leaks

Always free mnpt memory on failure in the zfs_unmount() function.

In the zfs_unshare_proto() function mountpoint is a const and
should not be assigned.

Signed-off-by: cao.xuewen <cao.xuewen@zte.com.cn>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #5054