pam: Fix "buffer overflow" in pam ZTS tests on F38

The pam ZTS tests were reporting a buffer overflow on F38, possibly
due to F38 now setting _FORTIFY_SOURCE=3 by default. gdb and
valgrind narrowed this down to a snprintf() buffer overflow in
zfs_key_config_modify_session_counter(). I'm not clear why this
particular snprintf() was being flagged as an overflow, but when
I replaced it with an asprintf(), the test passed reliably.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #14802
Closes #14842