Incorrect WriteAt implementation for mem.File

[tdakkota]

To reproduce: https://play.golang.org/p/-mM7CBvnFVl

I'd put fmt.Println(f.fileData.data) to (*mem.File).WriteAt, so I got

[0 0 0 0 0 1 2 3 4 5]
[1 2 3 4 5 1 2 3 4 5]
[1 2 3 4 5 1 2 3 4 5 1 2 3 4 5]
[0 0 0 0 0 1 2 3 4 5]
[0 0 0 0 0 1 2 3 4 5]

Seems like Write does not copy old data if reallocation needed.

[tdakkota]

Also, there is a possible race condition
Between

afero/mem/file.go

Line 283 in 32b5faa

atomic.StoreInt64(&f.at, off)

and

afero/mem/file.go

Line 261 in 32b5faa

cur := atomic.LoadInt64(&f.at)

f.at can be mutated