LU-15626 tests: Fix shellcheck error for rpc

LU-15626 tests: Fix shellcheck error for rpc

This patch addresses the errors and warnings
reported by shellcheck for rpc.sh. It also
breaks up the triple nested subshell for better
readability.

Signed-off-by: Timothy Day <timday@amazon.com>
Change-Id: I0d4afa83a6b9d4f825f31896a52dd30319b4bf51
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/49535
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>