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

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: https://github.com/KSPP/linux/issues/78
Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Reviewed-by: Paulo Alcantara <[e-mail]>
Signed-off-by: Steve French <[e-mail]>

fs/cifs/ntlmssp.h

···
 	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;
 
···
 	__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;
 
 /*