mkfile: use DATA_XFER_LIMIT

[mcimerman]

We should try to use the maximum allowed IPC transfer buffer size. Can possibly save 4 calls.

[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.

[mcimerman]

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.

I agree, but in the future if the block cache is improved and there will be buffering added, it'll help.

I will change it to literal (1024 * 64).