Add dynamic buffers to f3write

[AltraMayor]

The write buffer passed to the operating system has previously been limited to MAX_BUFFER_SIZE. This commit makes it dynamically sized in order to reach maximum writing speeds.

See details on issue #188.

[r2dd2]

I am testing the new f3read now. The previous f3read was already fast:
Validating file 3723.h2w ... 2097152/ 0/ 0/ 0
Validating file 3724.h2w ... 2097152/ 0/ 0/ 0
Validating file 3725.h2w ... 2097152/ 0/ 0/ 0
Validating file 3726.h2w ... 1766424/ 0/ 0/ 0

Data OK: 3.64 TB (7813657624 sectors)
Data LOST: 0.00 Byte (0 sectors)
Corrupted: 0.00 Byte (0 sectors)
Slightly changed: 0.00 Byte (0 sectors)
Overwritten: 0.00 Byte (0 sectors)
Average reading speed: 144.86 MB/s

I had run it to see if there were any errors shown as you said that the last bytes or buffers would not be written.

The current test should be ready in 8h as the files written with the dynamic buffer and flushing disabled are still on my drive.