why the extra work?

maybe the code has changed in all the intervening years?
   
   this is the virtually same as sunos_sys_mmap(), svr4_sys_mmap(),
   svr4_sys_mmap64(), and ultrix_sys_mmap().
   
   i can see why it might be done in svr4_32_sys_mmap(), maybe in
   svr4_32_sys_mmap64(), and sunos32_sys_mmap().  they're designed to
   operate in a mixed 32/64 bit environment, right?

not "mixed".  the _32 versions are are running 32 bit applications
in a 64 bit kernel environment.