LU-8465 ext2fs: parallel bitmap loading

LU-8465 ext2fs: parallel bitmap loading

In our benchmarking for PiB size filesystem, pass5 takes
10446s to finish and 99.5% of time takes on reading bitmaps.

It makes sense to reading bitmaps using multiple threads,
a quickly benchmark show 10446s to 626s with 64 threads.

Signed-off-by: Wang Shilong <wshilong@ddn.com>
Change-Id: I8d7389413a09bf262d0ae657cb485e8862385d0c
Reviewed-on: https://review.whamcloud.com/39851
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>