lib9p: Remove potential buffer overwrite in l9p_puqids()

Structure l9p_f_wralk reserves at most L9P_MAX_WELEM entries
and that number actually set the maximum we can safely use.

PR: 265385
Reviewed by: markj

(cherry picked from commit 2dd83b3f0507fc7bc64b908fb88f285a3b9663c8)