LU-15480 build: style cleanup lustre/tests/aiocp.c

LU-15480 build: style cleanup lustre/tests/aiocp.c

Add implicit-fallthrough decorator
Also some minor style-cleanups

Test-Parameters: trivial
HPE-bug-id: LUS-10201
Fixes: 689714eb51 ("LU-13846 llite: move iov iter forward by ourself")
Signed-off-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Change-Id: I3be3b27c1c006d13820ea55a84c083fbfc8c4b0f
Reviewed-on: https://review.whamcloud.com/46297
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Petros Koutoupis <petros.koutoupis@hpe.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>