devcoredump: avoid -Wempty-body warnings

devcoredump: avoid -Wempty-body warnings

Cleaning out the last -Wempty-body warnings found some interesting
cases with empty macros, along with harmless warnings like this one:

drivers/base/devcoredump.c: In function 'dev_coredumpm':
drivers/base/devcoredump.c:297:56: error: suggest braces around empty body in an 'if' statement [-Werror=empty-body]
  297 |                 /* nothing - symlink will be missing */;
      |                                                        ^
drivers/base/devcoredump.c:301:56: error: suggest braces around empty body in an 'if' statement [-Werror=empty-body]
  301 |                 /* nothing - symlink will be missing */;
      |                                                        ^

Randy tried addressing this one before, and there were multiple
other ideas in that thread.

Add a runtime warning and code comment here.

Link: https://lore.kernel.org/lkml/20200418184111.13401-8-rdunlap@infradead.org/
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20210322114258.3420937-1-arnd@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>