Rename fallthrough to zfs_fallthrough

Unfortunately macOS has obj-C keyword "fallthrough" in the OS headers.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Damian Szuberski <szuberskidamian@gmail.com>
Signed-off-by: Jorgen Lundman <lundman@lundman.net>
Closes #13097

module/lua/lstrlib.c

@@ -500,7 +500,7 @@ static const char *match (MatchState *ms, const char *s, const char *p) {
             }
             case '+':  /* 1 or more repetitions */
               s++;  /* 1 match already done */
-              fallthrough;
+              zfs_fallthrough;
             case '*':  /* 0 or more repetitions */
               s = max_expand(ms, s, p, ep);
               break;