hipFile: Mirror kernel's MAX_RW_COUNT computation

Since PAGE_SIZE is not the same on all plaforms, mirror the kernel's
computation of MAX_RW_COUNT so that hipFile doesn't use an incorrect value.

Author: kurtmcmillan

src/amd_detail/backend.h

@@ -12,15 +12,26 @@
 #include "sys.h"
 
 #include <cstddef>
+#include <climits>
 #include <memory>
 #include <sys/types.h>
 #include <unistd.h>
 
 namespace hipFile {
 
+static const size_t PAGE_SIZE{[] {
+    long v = sysconf(_SC_PAGE_SIZE);
+    if (v == -1) {
+        throw std::runtime_error("sysconf(_SC_PAGE_SIZE) failed");
+    }
+    return static_cast<size_t>(v);
+}()};
+
+static const size_t PAGE_MASK{~(PAGE_SIZE - 1)};
+
 // The maximum number of bytes that can be transferred in a single read() or
 // write() system call. Mirrors kernel's MAX_RW_COUNT
-static const size_t MAX_RW_COUNT = 0x7ffff000;
+static const size_t MAX_RW_COUNT = (INT_MAX & PAGE_MASK);
 
 struct Backend {
     virtual ~Backend() = default;