LU-15002 mke2fs: batch zeroing inode table

LU-15002 mke2fs: batch zeroing inode table

For flex_bg enabled fs, we could merge the
inode table blocks into a contiguous range,
this improves mke2fs time on large devices
when lazy_itable_init is disabled.

On a 977TB device, unpatched mke2fs was running
for 449m10s before getting terminated manually.
strace shows huge number of fallocate, given the
offset from fallocate it has done 41% of the inode
tables, the estimated time needed would be 1082m.

	unpatched 	patched
real	449m10.954s	4m20.531s
user	0m18.217s	0m16.147s
sys	0m20.311s	0m8.944s

Change-Id: I5257293ea35137cfc5321c15ed5dd8aa98c0612e
Signed-off-by: Li Dongyang <dongyangli@ddn.com>
Reviewed-on: https://review.whamcloud.com/c/tools/e2fsprogs/+/52215
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Emoly Liu <emoly@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>