selftests: netdevsim: Test psample functionality

Test various aspects of psample functionality over netdevsim and in
particular test that the psample module correctly reports the provided
metadata.

Example:

 # ./psample.sh
 TEST: psample enable / disable                                      [ OK ]
 TEST: psample group number                                          [ OK ]
 TEST: psample metadata                                              [ OK ]

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>