kmemcheck: add missing braces to do-while in kmemcheck_annotate_bitfield

Whether or not the sparse warning

	warning: do-while statement is not a compound statement

is justified or not in this case, it is annoying and trivial to fix.

[vegard.nossum@gmail.com: title and cleanup]
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Vegard Nossum <vegard.nossum@gmail.com>