LU-15220 utils: use 'fallthrough' pseudo keyword for switch

LU-15220 utils: use 'fallthrough' pseudo keyword for switch

'/* fallthrough */' hits implicit-fallthrough error with GCC 11.

This patch replaces the existing '/* fallthrough */' comments and
its variants with the 'fallthrough' pseudo keyword, which was added
by Linux kernel commit v5.4-rc2-141-g294f69e662d1.

Lustre-change: https://review.whamcloud.com/46270
Lustre-commit: TBD (from 6cae135625159733c0fc1b02ad0cd3ee25ff4d2b)

Test-Parameters: trivial
Change-Id: Icace4c9953950f86d3c48068d8c6bba7dd1160a7
Signed-off-by: Jian Yu <yujian@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/46276
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>