min_t type casting bug fixes

Hi,

We found a min_t type casting issue in fs/proc/vmcore.c, it uses smaller type in min_t so that during i386 PAE testing 64bit value was trucated and then cause saving vmcore failure and BUG() for mmap case.

I introduced new macros min_lt and max_lt to select the larger data type in x and y. Then use it in 2nd patch.

Any comments are appreciated.

Thanks
Dave