IB/uverbs: New macro to set pointers to NULL if length is 0 in INIT_UDATA()

Trying to have a ternary operator to choose between NULL (or 0) and the
real pointer value in invocations leads to an impossible choice between
a sparse error about a literal 0 used as a NULL pointer, and a gcc
warning about "pointer/integer type mismatch in conditional expression."

Rather than clutter the source with more casts, move the ternary
operator into a new INIT_UDATA_BUF_OR_NULL() macro, which makes it
easier to use and simplifies its callers.
Reported-by: Yann Droneaud <ydroneaud@opteya.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>

drivers/infiniband/core/uverbs.h

@@ -55,6 +55,14 @@
 		(udata)->outlen = (olen);				\
 	} while (0)
 
+#define INIT_UDATA_BUF_OR_NULL(udata, ibuf, obuf, ilen, olen)			\
+	do {									\
+		(udata)->inbuf  = (ilen) ? (const void __user *) (ibuf) : NULL;	\
+		(udata)->outbuf = (olen) ? (void __user *) (obuf) : NULL;	\
+		(udata)->inlen  = (ilen);					\
+		(udata)->outlen = (olen);					\
+	} while (0)
+
 /*
  * Our lifetime rules for these structs are the following:
  *

drivers/infiniband/core/uverbs_main.c

@@ -676,17 +676,14 @@ static ssize_t ib_uverbs_write(struct file *filp, const char __user *buf,
 				return -EINVAL;
 		}
 
-		INIT_UDATA(&ucore,
-			   (hdr.in_words) ? buf : 0,
-			   (unsigned long)ex_hdr.response,
-			   hdr.in_words * 8,
-			   hdr.out_words * 8);
-
-		INIT_UDATA(&uhw,
-			   (ex_hdr.provider_in_words) ? buf + ucore.inlen : 0,
-			   (ex_hdr.provider_out_words) ? (unsigned long)ex_hdr.response + ucore.outlen : 0,
-			   ex_hdr.provider_in_words * 8,
-			   ex_hdr.provider_out_words * 8);
+		INIT_UDATA_BUF_OR_NULL(&ucore, buf, (unsigned long) ex_hdr.response,
+				       hdr.in_words * 8, hdr.out_words * 8);
+
+		INIT_UDATA_BUF_OR_NULL(&uhw,
+				       buf + ucore.inlen,
+				       (unsigned long) ex_hdr.response + ucore.outlen,
+				       ex_hdr.provider_in_words * 8,
+				       ex_hdr.provider_out_words * 8);
 
 		err = uverbs_ex_cmd_table[command](file,
 						   &ucore,