selftests/powerpc: Move perror closer to its use

Right now, if perf_event_open() fails for the systemwide tests, error
report is printed too late, sometimes after subsequent system calls.
Move use of perror() to the main function, just after the syscall.

Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/372ac78c27899f1f612fbd6ac796604a4a9310aa.1669096083.git.naveen.n.rao@linux.vnet.ibm.com