Fix 'zpool import' detection issue

Fix 'zpool import' detection issue

Before adding the entry to the configuration verify that the
device can be opened exclusively.  This ensures that as long
as multipathd is running the underlying multipath devices, which
otherwise appear identical to their /dev/mapper counterpart,
are pruned from the configuration.

Failure to do so can result in a result in the vdev appearing
as UNAVAIL when the vdev path provided to the kernel can't be
opened exclusively.

This check would normally be performed in zpool_open_func()
but placing it there would result in false positives because
it is called concurrently for many devices.

Reviewed-by: Olaf Faaland <faaland1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #5387