X Provide mlock2 fallback for Ubuntu

which has glibc 2.23 but mlock2 was added only to glibc 2.27.
The kernel (4.4.x) has mlock2 though.

wcfs/internal/mm.pyx

@@ -25,6 +25,30 @@ from posix cimport mman
 from cpython.exc cimport PyErr_SetFromErrno
 #from libc.stdio cimport printf
 
+# mlock2 is provided starting from glibc 2.27
+cdef extern from *:
+    """
+    #if defined(__GLIBC__)
+    #if !__GLIBC_PREREQ(2, 27)
+    #include <unistd.h>
+    #include <sys/syscall.h>
+    static int mlock2(const void *addr, size_t len, int flags) {
+        long err = syscall(SYS_mlock2, addr, len, flags);
+        if (err != 0) {
+            errno = -err;
+            return -1;
+        }
+        return 0;
+    }
+    #endif
+    #endif
+
+    #ifndef MLOCK_ONFAULT
+    # define MLOCK_ONFAULT  1
+    #endif
+    """
+    pass
+
 cdef extern from "<sys/user.h>":
     cpdef enum:
         PAGE_SIZE
@@ -40,7 +64,7 @@ cpdef enum:
     MADV_SEQUENTIAL = mman.MADV_SEQUENTIAL
     MADV_WILLNEED   = mman.MADV_WILLNEED
     MADV_DONTNEED   = mman.MADV_DONTNEED
-    MADV_FREE       = mman.MADV_FREE
+    #MADV_FREE       = mman.MADV_FREE
     MADV_REMOVE     = mman.MADV_REMOVE
 
     MS_ASYNC        = mman.MS_ASYNC