dd: seek=0 still seeks, erroring on non-seekable files

Hi!

The Linux kernel selftest for ftrace:trace_marker_raw is failing on Ubuntu 26.04 because of a parity issue in dd between rust coreutils and GNU coreutils. A call to dd was added to the Linux test in this commit, and the output file is trace_marker_raw, which notably does not support seeking. With GNU dd, this test passes, and with uutils dd the test hangs until timeout.

I believe this to be because of a few spots where dd unconditionally seeks, and errors from non-seekable files are not handled. Here's a minimal reproducer for a system with dd pointing to the GNU binary:

#!/bin/sh
echo "GNU:"
echo hello | dd of=/sys/kernel/tracing/trace_marker_raw
echo "uutils:"
echo hello | coreutils/target/debug/coreutils dd of=/sys/kernel/tracing/trace_marker_raw

which outputs:

GNU:
0+1 records in
0+1 records out
6 bytes copied, 2.6711e-05 s, 225 kB/s
uutils:
dd: failed to seek in output file: Illegal seek

Start by reproducing the shell pipeline against /sys/kernel/tracing/trace_marker_raw and compare the uutils dd behavior with GNU dd. Trace the dd implementation's unconditional seek operations and adjust the behavior so non-seekable output succeeds like GNU dd without hanging or reporting Illegal seek.