Possible Valgrind issue in `dt_write`

$ test/unittest/dtrace-util/tst.ELFGenerationOut.sh /tmp/runtest.10198/bin/dtrace
--149025-- WARNING: unhandled eBPF command 23
==149025== Syscall param write(buf) points to uninitialised byte(s)
==149025==    at 0x4AF38D0: write (write.c:26)
==149025==    by 0x491460B: dt_write (dt_subr.c:497)
==149025==    by 0x48E2286: dump_elf64 (dt_link.c:725)
==149025==    by 0x48E2286: dtrace_program_link (dt_link.c:1615)
==149025==    by 0x10A0E7: link_prog (dtrace.c:408)
==149025==    by 0x10A0E7: main (dtrace.c:1419)
==149025==  Address 0x9390b34 is 260 bytes inside a block of size 683 alloc'd
==149025==    at 0x485032F: calloc (vg_replace_malloc.c:1675)
==149025==    by 0x4914C34: dt_zalloc (dt_subr.c:685)
==149025==    by 0x48AB05C: dt_buf_create (dt_buf.c:30)
==149025==    by 0x48D4245: dtrace_dof_create (dt_dof.c:824)
==149025==    by 0x48DF919: dtrace_program_link (dt_link.c:1570)
==149025==    by 0x10A0E7: link_prog (dtrace.c:408)
==149025==    by 0x10A0E7: main (dtrace.c:1419)
==149025==

Run test/unittest/dtrace-util/tst.ELFGenerationOut.sh with the reported dtrace binary under Valgrind. Start by tracing the dt_write call in dt_subr.c and the ELF-generation path through dt_link.c, dtrace.c, dt_dof.c, and dt_buf.c. Done means identifying whether the write uses uninitialised bytes and resolving or documenting the warning with a verified test result.