LU-15492 build: fallthrough macro for pre/post gcc-7

LU-15492 build: fallthrough macro for pre/post gcc-7

gcc-7.5 on openSUSE 15:
   error: this statement may fall through [-Werror=implicit-fallthrough=]

Use the __attribute__((fallthrough)) for gcc-7 and later.
and use a no op statement for earlier gcc where the fallthrough
attribute is not available.

Lustre-change: https://review.whamcloud.com/46357
Lustre-commit: TBD (from 48145f2eaed1537506bfffb1c0a44a8cfdb38254)

Test-Parameters: trivial
Fixes: 5549b1b9e0 ("LU-15220 lustre: use 'fallthrough' pseudo keyword for switch")
Signed-off-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Change-Id: Ib72f5996149c738805f15e354e1e1606d981ce29
Reviewed-on: https://review.whamcloud.com/46643
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>