kernel/sys: annotate implicit fall through

There is a plan to build the kernel with -Wimplicit-fallthrough and this
place in the code produced a warning (W=1).

This commit remove the following warning:

  kernel/sys.c:1748:6: warning: this statement may fall through [-Wimplicit-fallthrough=]

Link: http://lkml.kernel.org/r/20190114203347.17530-1-malat@debian.org


Signed-off-by: Mathieu Malaterre <malat@debian.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>