I'm not sure when this sort happens, but early boot may not have a proper virtual memory implementation yet, so the kernel has to use a fixed stack size allocated by the boot loader.
Right, in this case specifically we're running off a limited bootstack allocated in the kernel's .bss (somewhere between 2 to 6 pages, generally); we won't finish initializing VM until shortly after this sort is done (in some of the SYSINITs that we're sorting).