io_u: only rewind file position if it's non-zero

This:

bs=8k
rw=read:-4k

is supposed to read 0..8k, then add 8k and subtract 4k, ending
up with a next read at 4k..12k and so forth. But we rewind too
quickly, and the first IO fails as being out-of-bounds.

Fixes: c22825bb537af ("Fix backwards reads with --size smaller than the file size")
Signed-off-by: Jens Axboe <axboe@kernel.dk>