ldisc_deregister missing lower-bound check: negative index OOB write into linesw[]

Summary

ldisc_register validates discipline>=0&&<MAXLDISC(:105). ldisc_deregister only checks <MAXLDISC(:126). Signed int negative passes -> linesw[discipline]=nodisc(:127) writes before array. Kernel module callers only (root). Asymmetric with register. Defense-in-depth.