LU-14507 mdt: handle default stripe_count=-1 properly

LU-14507 mdt: handle default stripe_count=-1 properly

If the default LMV stripe_count=-1 print it as a signed value
instead of unsigned, to better match how it is set with "-c -1".

Lustre-commit: d9753b5ba6ad29fd8958a47b462d2fa594ba1145
Lustre-change: https://review.whamcloud.com/41983

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I106f266c33e2c2cf0f5bcc1491e4bc5ac93ebbe5
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/43147
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Li Xi <lixi@ddn.com>