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A compilation -Wimplicit-fallthrough warning was enabled by commit
a035d552a93b ("Makefile: Globally enable fall-through warning")
and triggers the following warning.

fs/aufs/opts.h:78:11:
warning: this statement may fall through [-Wimplicit-fallthrough=]

This patch adds comments according GNU manual.
https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#Warning-Options

Signed-off-by: He Zhe <zhe.he@windriver.com>
See-also: https://www.mail-archive.com/aufs-users@lists.sourceforge.net/msg05685.html
(cherry picked from commit e7619996b014c5d8fd2f6ad89542c545a207667f)