Quiet valgrind complaints following pread/pwrite changes

Per complaints from buildfarm and elsewhere
Patch from Jasper Pedersen
Discussion: https://postgr.es/m/0f419c91-49ab-2399-0143-13063bd97c46@redhat.com

src/tools/valgrind.supp

@@ -52,9 +52,10 @@
 {
 	padding_XLogRecData_write
 	Memcheck:Param
-	write(buf)
+	pwrite64(buf)
 
 	...
+	fun:pwrite
 	fun:XLogWrite
 }