perf test shell: Replace '|&' with '2>&1 |' to work with more shells

Since we do not specify bash (and/or zsh) as a requirement, use the
standard error redirection that is more widely supported.

Signed-off-by: Kim Phillips <kim.phillips@arm.com>
Link: http://lkml.kernel.org/n/tip-ji5mhn3iilgch3eaay6csr6z@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>