LU-12635 lnet: Fix style issues for selftest/rpc.c

LU-12635 lnet: Fix style issues for selftest/rpc.c

This patch fixes issues reported by checkpatch for the file
selftest/rpc.c.
Linux 5.3 enforces the use of 'fallthrough' which is also
suggested by checkpatch

Test-Parameters: trivial
Cray-bug-id: LUS-7690
Signed-off-by: Shaun Tancheff <stancheff@cray.com>
Change-Id: I049e32c0b0cf1002166445a89ac39110442d28bd
Reviewed-on: https://review.whamcloud.com/35800
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Petros Koutoupis <pkoutoupis@cray.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>