checkpatch: add a placeholder to check blank lines before declarations

Figure out first how to determine if this is in a struct declaration or in
a function body before enabling this.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>