LU-18114 lfsck: split "lctl lfsck_*" group into subcommands

LU-18114 lfsck: split "lctl lfsck_*" group into subcommands

Split "lctl lfsck_*" command group into subcommands, e.g.
"lctl lfsck_start" to "lctl lfsck start".
Also, sanity-lfsck.sh test_12a is modified a little to verify
this patch.

Signed-off-by: Emoly Liu <emoly@whamcloud.com>
Change-Id: I039f8942b45a16a26386b579919018163e0d8c7c
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/55997
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Frederick Dilger <fdilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>