Partially revert 24d0e21f51e88ab35cc3f1c1750741dfeb40933d

This reverts the extra for loop condition from
"do_iterative_fd_io_iterate(): Support sparse files".

When reading a file with more than 8 block_runs, get_vecs() would
return B_BUFFER_OVERFLOW which would never create any subrequest due
to the test on error == B_OK on the loop, but instead just fail.

Except for the get_vecs() return code, where it is not wanted,
the test made no sense as all other assignments are tested directly
or passed around with break.

Works for me but I don't guarantee it's completely correct.