add mlock2 linux syscall wrapper

mlock2 syscall was added in linux v4.4 and glibc has api for it.
It falls back to mlock in case of flags==0, so that case works
even on older kernels.

MLOCK_ONFAULT is moved under _GNU_SOURCE following glibc.

include/sys/mman.h

@@ -94,6 +94,13 @@ extern "C" {
 #define MADV_SOFT_OFFLINE 101
 #endif
 
+#ifdef _GNU_SOURCE
+#define MREMAP_MAYMOVE 1
+#define MREMAP_FIXED 2
+
+#define MLOCK_ONFAULT 0x01
+#endif
+
 #include <bits/mman.h>
 
 void *mmap (void *, size_t, int, int, int, off_t);
@@ -110,14 +117,12 @@ int mlockall (int);
 int munlockall (void);
 
 #ifdef _GNU_SOURCE
-#define MREMAP_MAYMOVE 1
-#define MREMAP_FIXED 2
 void *mremap (void *, size_t, size_t, int, ...);
 int remap_file_pages (void *, size_t, int, size_t, int);
+int mlock2 (const void *, size_t, unsigned);
 #endif
 
 #if defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
-#define MLOCK_ONFAULT   0x01
 int madvise (void *, size_t, int);
 int mincore (void *, size_t, unsigned char *);
 #endif

src/linux/mlock2.c

+#define _GNU_SOURCE 1
+#include <sys/mman.h>
+#include "syscall.h"
+
+int mlock2(const void *addr, size_t len, unsigned flags)
+{
+	if (flags == 0)
+		return mlock(addr, len);
+	return syscall(SYS_mlock2, addr, len, flags);
+}