kernel: ppp_async: limit MRU to 64K

In the Linux kernel, the following vulnerability has been resolved: pppasync: limit MRU to 64K syzbot triggered a warning 1 in allocpages: WARNONONCEGFPorder MAXPAGEORDER, gfp Willem fixed a similar issue in commit c0a2a1b0d631 "ppp: limit MRU to 64K" Adopt the same sanity check for...