Fix zpool_add_005_pos

Fix zpool_add_005_pos

Under Linux the existence of a block device in /etc/fstab is
not sufficient to prevent the use of the force flag.  Without
the force flag a warning will be printed that the device has
a filesystem of a given type.  Providing the force option
will overwrite that filesystem as long as it is not actively
mounted.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Tested-by: bunder2015 <omfgbunder@gmail.com>
Signed-off-by: bunder2015 <omfgbunder@gmail.com>
Closes #6267
Closes #6272