checkpatch: simplify and consolidate "missing space after" checks

Commonise the code for missing spaces after struct, union, and enum such
that they share the same code.  Ensure we cover all the common cases in
each case.  Check against the sanitised line to ensure we do not report on
comments and strings.

Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>