RamFSFile read/write does not update file seek offset

[Woazboat]

The code for RamFSFile::read/write should be identical to MinixFSFile::read/write, which correctly updates the file seek offset after read/write.

RamFSFile:
return (f_inode_->readData(offset_ + offset, count, buffer));

MinixFSFile:

int32 read_bytes = f_inode_->readData(offset_ + offset, count, buffer);
offset_ += read_bytes;
return read_bytes;

The cleanest solution would be to move the implementation to the shared 'File' base class, since it's identical for all file systems.

[Woazboat]

Additionally, the update of the file seek offset is not safe for concurrent calls to read/write/lseek using the same file descriptor.

offset_ = 0

1. readData(offset_ = 0, 5 bytes, ...)
2. readData(offset_ = 0, 5 bytes, ...)
3. offset_ += 5
4. offset_ += 5

=> offset_ = 10, but bytes 5-9 were never read