nvpair: Use flexible array member for nvpair name strings

Coverity reported possible out-of-bounds reads from doing `((char
*)(nvp) + sizeof (nvpair_t))` to get the nvpair name string. These were
initially marked as false positives, but since we are now using C99
flexible array members elsewhere, we could use them here too as cleanup
to make the code easier to understand.

Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reported-by: Coverity (CID-977165)
Reported-by: Coverity (CID-1524109)
Reported-by: Coverity (CID-1524642)
Closes openzfs#14612

include/sys/nvpair.h

@@ -76,7 +76,7 @@ typedef struct nvpair {
 	int16_t	nvp_reserve;	/* not used */
 	int32_t	nvp_value_elem;	/* number of elements for array types */
 	data_type_t nvp_type;	/* type of value */
-	/* name string */
+	char	nvp_name[];	/* name string */
 	/* aligned ptr array for string arrays */
 	/* aligned array of data for value */
 } nvpair_t;
@@ -109,7 +109,7 @@ typedef struct nvlist {
 #define	NV_ALIGN4(x)		(((x) + 3) & ~3)
 
 #define	NVP_SIZE(nvp)		((nvp)->nvp_size)
-#define	NVP_NAME(nvp)		((char *)(nvp) + sizeof (nvpair_t))
+#define	NVP_NAME(nvp)		((nvp)->nvp_name)
 #define	NVP_TYPE(nvp)		((nvp)->nvp_type)
 #define	NVP_NELEM(nvp)		((nvp)->nvp_value_elem)
 #define	NVP_VALUE(nvp)		((char *)(nvp) + NV_ALIGN(sizeof (nvpair_t) \