cifs: Replace zero-length arrays with flexible-array members

BugLink: https://bugs.launchpad.net/bugs/2036450

Zero-length arrays are deprecated[1] and we are moving towards
adopting C99 flexible-array members instead. So, replace zero-length
arrays in a couple of structures with flex-array members.

This helps with the ongoing efforts to tighten the FORTIFY_SOURCE
routines on memcpy() and help us make progress towards globally
enabling -fstrict-flex-arrays=3 [2].

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#zero-length-and-one-element-arrays [1]
Link: https://gcc.gnu.org/pipermail/gcc-patches/2022-October/602902.html [2]
Link: KSPP/linux#78
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Paulo Alcantara <pc@cjr.nz>
Signed-off-by: Steve French <stfrench@microsoft.com>
(cherry picked from commit 4e551db)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

fs/cifs/ntlmssp.h

@@ -83,7 +83,7 @@ typedef struct _NEGOTIATE_MESSAGE {
 	SECURITY_BUFFER WorkstationName;	/* RFC 1001 and ASCII */
 	/* SECURITY_BUFFER for version info not present since we
 	   do not set the version is present flag */
-	char DomainString[0];
+	char DomainString[];
 	/* followed by WorkstationString */
 } __attribute__((packed)) NEGOTIATE_MESSAGE, *PNEGOTIATE_MESSAGE;
 
@@ -135,7 +135,7 @@ typedef struct _AUTHENTICATE_MESSAGE {
 	__le32 NegotiateFlags;
 	/* SECURITY_BUFFER for version info not present since we
 	   do not set the version is present flag */
-	char UserString[0];
+	char UserString[];
 } __attribute__((packed)) AUTHENTICATE_MESSAGE, *PAUTHENTICATE_MESSAGE;
 
 /*