LU-16169 e2fsck: improve parallel thread balance

LU-16169 e2fsck: improve parallel thread balance

Improve the balance of work allocated to each thread by distributing
a more equal number of inodes to each thread, rather than an equal
number of groups.

In some cases in real usage, the number of inodes allocated to
threads with equal numbers of groups can vary by 10x or more, which
leads to pass1 threads having a runtime time roughly propritional
to the number of inodes allocated to them.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Iea964cca33d19170e9b6d88aa725dc878cae6ce2
Reviewed-on: https://review.whamcloud.com/c/tools/e2fsprogs/+/48806
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Li Dongyang <dongyangli@ddn.com>