compiler_attributes.h: Add 'fallthrough' pseudo keyword for switch/case use

compiler_attributes.h: Add 'fallthrough' pseudo keyword for switch/case use

Reserve the pseudo keyword 'fallthrough' for the ability to convert the
various case block /* fallthrough */ style comments to appear to be an
actual reserved word with the same gcc case block missing fallthrough
warning capability.

All switch/case blocks now should end in one of:

	break;
	fallthrough;
	goto <label>;
	return [expression];
	continue;

In C mode, GCC supports the __fallthrough__ attribute since 7.1,
the same time the warning and the comment parsing were introduced.

fallthrough devolves to an empty "do {} while (0)" if the compiler
version (any version less than gcc 7) does not support the attribute.

Signed-off-by: Joe Perches <[email protected]>
Acked-by: Peter Zijlstra (Intel) <[email protected]>
Suggested-by: Dan Carpenter <[email protected]>
Cc: Miguel Ojeda <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>