LU-13210 lnet: gcc8 add implicit-fallthrough decorator

LU-13210 lnet: gcc8 add implicit-fallthrough decorator

With newer compilers and newer kernels -Werror=implicit-fallthrough
is enabled.

This adds the missing decorator.

Test-Parameters: trivial
Cray-bug-id: LUS-8476
Signed-off-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Change-Id: I47334d5a8d0bcf17489c1b15af29cd553fa01a09
Reviewed-on: https://review.whamcloud.com/37466
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Neil Brown <neilb@suse.de>
Reviewed-by: Petros Koutoupis <petros.koutoupis@hpe.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Wang Shilong <wshilong@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>