fix write volume over size MaxPossibleVolumeSize

[kungf]

What problem are we solving?

volume over size MaxPossibleVolumeSize
#5182

How are we solving the problem?

1. when write, n.Size is not init yet before prepareWriteBuf, so n.Size.IsValid() alaways false!
2. v.nm.ContentSize() only have write data use size not include delete use size, so if volume has many delete ops(n) before full, more than n*32 bytes can also write to volume, even volume true offset is large than MaxPossibleVolumeSize

How is the PR tested?

Checks

• I have added unit tests if possible.
• I will add related wiki document changes and link to this PR after merging.

[chrislusf]

Thanks for the findings and the PR!

• How is this PR tested?
• Deletion can still work if over the size limit?
• This PR is functionally the same as before. What is the difference this can make related to [Critical] volume write more than MaxPossibleVolumeSize bytes, case vacuuming size mismatch #5182 ?

[kungf]

hi @chrislusf，I am glad to be able to do something for SeaweedFS！

• How is this PR tested?

I will test it by these steps：

1. write some files
2. delete these files
3. repeated 1,2 until volume is full
   I wrote a tool to check whether needs to set over flow。

• Deletion can still work if over the size limit?

Yes， If we disable deletion after the volume is full， it will be unable to reclaim space。

• This PR is functionally the same as before. What is the difference this can make related to [Critical] volume write more than MaxPossibleVolumeSize bytes, case offset truncate #5182?

when we only check size by “MaxPossibleVolumeSize < v.nm.ContentSize()+uint64(actualSize)”，and v.nm.ContentSize() only includes write data size but does not include delete op size, so if the volume had many deletions, even " MaxPossibleVolumeSize < v.nm.ContentSize()+uint64(actualSize)" is false, write needle offset can also exceed MaxPossibleVolumeSize。

This pr can fix size mismatch err when vacuuming。