cp: use DATA_XFER_LIMIT

[mcimerman]

No description provided.

[jxsvoboda]

Increasing the buffer size is probably OK, although it will typically not bring huge improvement in the typical scenario as the bottleneck is between the file system and block device, with much smaller buffers.

If you want to do this, then either just continue using a literal constant, or have vfs define some constant, e.g. VFS_XFER_SIZE to make this nicely encapsulated. We don't want to leak the details of how the VFS interface works by tapping directly into IPC stuff.