lib: Ignore ENOSYS errors from madvise() calls.

For example Raspberry Pi kernels have removed this. We'll wrap all the
madvise() calls with my_madvise() to avoid extra checks all over the place.

src/lib/mmap-util.c

@@ -37,13 +37,18 @@ void *mmap_rw_file(int fd, size_t *length)
 	return mmap_file(fd, length, PROT_READ | PROT_WRITE);
 }
 
-#ifndef HAVE_MADVISE
+#undef madvise
 int my_madvise(void *start ATTR_UNUSED, size_t length ATTR_UNUSED,
 	       int advice ATTR_UNUSED)
 {
+#ifdef HAVE_MADVISE
+	/* Ignore ENOSYS errors, which happen if the kernel hasn't implemented
+	   the syscall even if libc has. */
+	if (madvise(start, length, advice) < 0 && errno != ENOSYS)
+		return -1;
+#endif
 	return 0;
 }
-#endif
 
 size_t mmap_get_page_size(void)
 {

src/lib/mmap-util.h

@@ -14,9 +14,9 @@
 #  define MREMAP_MAYMOVE 1
 #endif
 
-#ifndef HAVE_MADVISE
-#  define madvise my_madvise
+#define madvise my_madvise
 int my_madvise(void *start, size_t length, int advice);
+#ifndef HAVE_MADVISE
 #  ifndef MADV_NORMAL
 #    define MADV_NORMAL 0
 #    define MADV_RANDOM 0