treewide: Use fallthrough pseudo-keyword

Replace the existing /* fall through */ comments and its variants with
the new pseudo-keyword macro fallthrough[1]. Also, remove unnecessary
fall-through markings when it is the case.

[1] https://www.kernel.org/doc/html/v5.7/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-through

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
diff --git a/fs/cifs/smb2pdu.c b/fs/cifs/smb2pdu.c
index 667d70a..96c172d 100644
--- a/fs/cifs/smb2pdu.c
+++ b/fs/cifs/smb2pdu.c
@@ -1101,7 +1101,7 @@ smb2_select_sectype(struct TCP_Server_Info *server, enum securityEnum requested)
 		if ((server->sec_kerberos || server->sec_mskerberos) &&
 			(global_secflags & CIFSSEC_MAY_KRB5))
 			return Kerberos;
-		/* Fallthrough */
+		fallthrough;
 	default:
 		return Unspecified;
 	}