LU-8465 e2fsck: avoid too much memory allocation for pfsck

LU-8465 e2fsck: avoid too much memory allocation for pfsck

e2fsck init memory according to filesystem inodes/dir numbers
recorded in the superblock, this should be aware of filesystem
number of threads, otherwise, oom happen.

Signed-off-by: Wang Shilong <wshilong@ddn.com>
Change-Id: I99b7415231e0eb3670acec82a8a7b85b20dd36d5
Reviewed-on: https://review.whamcloud.com/39849
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>