Linux 6.10 compat: Fix tracepoints definitions

string field definition includes the source variable for a value
of the string when the TP hits; in 6.10+ kernels, assign_str()
uses that to copy a value from src to the string, with older
kernels, __assign_str still accepted src as a second parameter.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Pavel Snajdr <snajpa@snajpa.net>
Co-authored-by: Tony Hutter <hutter2@llnl.gov>
Closes #16475
Closes #16515