sheep: remove last slash for new->path in md_add_disk()

Follow these steps:

  1. sheep /meta/,/data1/,/data2/...... (add slash before data path)
  2. upload data
  3. remove /data1/*
  4. get data

then the sheep daemon will go to dead loops and print logs like:

  ERROR [io 1234] err_to_sderr(96) /data1/ corrupted
  ERROR [io 1234] err_to_sderr(96) /disk5/ corrupted
  ...

The reason is when the sheep daemon start, it use "/data1/" as disk->path, but
when it occur a error-io, sheep will call md_remove_disk() with argument disk which
contain path of "/data1" (without slash), so sheep can not actually unplug "/data1/"
in md ring and it will try to forward request forever.

We can fix this by remove last slash when sheep call md_add_disk().

Signed-off-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Meng An <isolate000@gmail.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>