minmax: simplify and clarify min_t()/max_t() implementation

minmax: simplify and clarify min_t()/max_t() implementation

This simplifies the min_t() and max_t() macros by no longer making them
work in the context of a C constant expression.

That means that you can no longer use them for static initializers or
for array sizes in type definitions, but there were only a couple of
such uses, and all of them were converted (famous last words) to use
MIN_T/MAX_T instead.

Cc: David Laight <David.Laight@aculab.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>