busybox

dd: fix handling of short result of full_write(), closes 11711

$ dd bs=1G <sda1 of=/dev/sda1
dd: error writing '/dev/sda1': No space left on device
1+0 records in
0+0 records out
999292928 bytes (953.0MB) copied, 0.784617 seconds, 1.2GB/s

function                                             old     new   delta
write_and_stats                                       99     102      +3

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>