selftests: fib_tests: Count all trace point invocations

The tests rely on the IPv{4,6} FIB trace points being triggered once for
each forwarded packet. If receive processing is deferred to the
ksoftirqd task these invocations will not be counted and the tests will
fail. Fix by specifying the '-a' flag to avoid perf from filtering on
the mausezahn task.

Before:

 # ./fib_tests.sh -t ipv4_mpath_list

 IPv4 multipath list receive tests
     TEST: Multipath route hit ratio (.68)                               [FAIL]

 # ./fib_tests.sh -t ipv6_mpath_list

 IPv6 multipath list receive tests
     TEST: Multipath route hit ratio (.27)                               [FAIL]

After:

 # ./fib_tests.sh -t ipv4_mpath_list

 IPv4 multipath list receive tests
     TEST: Multipath route hit ratio (1.00)                              [ OK ]

 # ./fib_tests.sh -t ipv6_mpath_list

 IPv6 multipath list receive tests
     TEST: Multipath route hit ratio (.99)                               [ OK ]

Fixes: 8ae9efb8

 ("selftests: fib_tests: Add multipath list receive tests")
Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/netdev/202309191658.c00d8b8-oliver.sang@intel.com/
Tested-by: kernel test robot <oliver.sang@intel.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Tested-by: Sriram Yagnaraman <sriram.yagnaraman@est.tech>
Link: https://lore.kernel.org/r/20231010132113.3014691-3-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>