LINUX 5.3: Add comments for fallthrough switch cases

LINUX 5.3: Add comments for fallthrough switch cases

With commit 6e0f1c3b45102e7644d25cf34395ca980414317f (LINUX: Honor
--enable-checking for libafs) building libafs against a linux 5.3
kernel compiles with errors due to fall through in case statements when
--enable-checking / --enable-warning is used.

e.g.
  src/opr/jhash.h:82:17: error: this statement may fall through
                                [-Werror=implicit-fallthrough=]
         case 3 : c+=k[2];
                  ~^~~~~~

The GCC compiler will disable the implicit-fallthrough check for case
statements that contain a "special" comment ( /* fall through */ ).

Add the 'fall through' comment to indicate where fall throughs are
acceptable.

This commit only adds comments and does not alter any executable code.

The -Wimplicit-fallthrough flag was enabled globally in the linux kernel
build in 5.3-rc2 (commit: a035d552a93bb9ef6048733bb9f2a0dc857ff869
Makefile: Globally enable fall-through warning)

Reviewed-on: https://gerrit.openafs.org/13881
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit a455452d7ee98d160620925bb8a0e3d0f4dfd7ec)

Change-Id: Icad4b5923d971e7519f5d5259cd9c009c40c0d7a
Reviewed-on: https://gerrit.openafs.org/13910
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>