selftests: pmtu.sh: Remove quotes around commands in setup_xfrm

The first command in setup_xfrm is failing resulting in the test getting
skipped:

+ ip netns exec ns-B ip -6 xfrm state add src fd00:1::a dst fd00:1::b spi 0x1000 proto esp aead 'rfc4106(gcm(aes))' 0x0f0f0f0f0f0f0f0f0f0f0f0f0f0f0f0f0f0f0f0f 128 mode tunnel
+ out=RTNETLINK answers: Function not implemented
...
  xfrm6 not supported
TEST: vti6: PMTU exceptions                                         [SKIP]
  xfrm4 not supported
TEST: vti4: PMTU exceptions                                         [SKIP]
...

The setup command started failing when the run_cmd option was added.
Removing the quotes fixes the problem:
...
TEST: vti6: PMTU exceptions                                         [ OK ]
TEST: vti4: PMTU exceptions                                         [ OK ]
...

Fixes: 56490b62 ("selftests: Add debugging options to pmtu.sh")
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>