selftests/net: convert pmtu.sh to run it in unique namespace

pmtu test use /bin/sh, so we need to source ./lib.sh instead of lib.sh
Here is the test result after conversion.

 # ./pmtu.sh
 TEST: ipv4: PMTU exceptions                                         [ OK ]
 TEST: ipv4: PMTU exceptions - nexthop objects                       [ OK ]
 TEST: ipv6: PMTU exceptions                                         [ OK ]
 TEST: ipv6: PMTU exceptions - nexthop objects                       [ OK ]
 ...
 TEST: ipv4: list and flush cached exceptions - nexthop objects      [ OK ]
 TEST: ipv6: list and flush cached exceptions                        [ OK ]
 TEST: ipv6: list and flush cached exceptions - nexthop objects      [ OK ]
 TEST: ipv4: PMTU exception w/route replace                          [ OK ]
 TEST: ipv4: PMTU exception w/route replace - nexthop objects        [ OK ]
 TEST: ipv6: PMTU exception w/route replace                          [ OK ]
 TEST: ipv6: PMTU exception w/route replace - nexthop objects        [ OK ]

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>