bugfix: dfget overwrite /dev/null

[xuyixin1996]

Ⅰ. Describe what this PR did

1. bugfix: dfget will overwrite the /dev/null and caused some issue
2. bugfix: supernode will panic when receive metrics report from dfget which the fileLength is -1 (default value)

Ⅱ. Does this pull request fix one issue?

N.A

Ⅲ. Why don't you add test cases (unit test/integration test)? (你真的觉得不需要加测试吗？)

N.A

Ⅳ. Describe how to verify it

dfget -u http://google.com -o /dev/null
ls /dev/null

before the patch, /dev/null will be a regular file
after the patch, /dev/null will not be changed

Ⅴ. Special notes for reviews

[pouchrobot]

We found this is your first time to contribute to Dragonfly, @xuyixin1996
👏 We really appreciate it.
Just remind that you have read the contribution guide: https://github.com/dragonflyoss/Dragonfly/blob/master/CONTRIBUTING.md
If you didn't, you should do that first. If done, welcome again and please enjoy hacking! 🍻

[codecov-io]

Codecov Report

Merging #1254 into master will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1254      +/-   ##
==========================================
- Coverage   48.43%   48.41%   -0.02%     
==========================================
  Files         119      119              
  Lines        7573     7578       +5     
==========================================
+ Hits         3668     3669       +1     
- Misses       3608     3612       +4     
  Partials      297      297              

Impacted Files	Coverage Δ
dfget/core/downloader/downloader.go	86.11% <0.00%> (-13.89%)	⬇️
supernode/daemon/mgr/scheduler/manager.go	22.60% <0.00%> (+0.68%)	⬆️

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[allencloud]

Please do not edit this file directory, you should update swagger.yml first, and then auto-generate this file:
https://github.com/dragonflyoss/Dragonfly/blob/master/apis/README.md

[lowzj]

I'm wondering in which scenario the flag --output should be set as /dev/null.
It may change the following codes to support writing /dev/null:

• dfget/core#prepare: don't create target directory and temporary file
• /dfget/core/downloader/p2p_downloader#TargetWriter: it should not be run in this case

[ML99bug]

😄😄😄😄

[allencloud]

@xuyixin1996 conflict happens, could you help to rebase to solve it? Thanks

[pouchrobot]

ping @xuyixin1996
Conflict happens after merging a previous commit.
Please rebase the branch against master and push it back again. Thanks a lot.