LU-8465 e2fsck: misc cleanups for pfsck

LU-8465 e2fsck: misc cleanups for pfsck

Add -m option description to e2fsck.8 man page.

Rename e2fsck_struct fs_num_threads to pfs_num_threads to avoid
confusion with the ext2_filsys fs_num_threads field, and move
thread_info to be together with the other CONFIG_PFSCK fields.

Move ext2_filsys fs_num_threads to fit into the __u16 "pad" field
to avoid consuming one of the few remaining __u32 reserved fields.

Fix a few print format warnings.

Change-Id: Ie69fb51642e71ca5d0d242d9cdc12d29ff181f52
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/40075
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Wang Shilong <wshilong@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>