Add a new section to the CodingStyle file, encouraging people not to
re-invent available kernel macros such as ARRAY_SIZE(), FIELD_SIZEOF(),
min() and max(), among others.

Signed-off-by: Robert P. J. Day <rpjday@mindspring.com>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: Jan Engelhardt <jengelh@gmx.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>