Hide statfs/fstatfs' ST_VALID flag from userspace.

Spotted while debugging the strace 4.10 upgrade.

Change-Id: I1af1be9c9440151f55f74a835e1df71529b0e4fe
diff --git a/libc/arch-arm64/syscalls/__fstatfs.S b/libc/arch-arm64/syscalls/__fstatfs.S
new file mode 100644
index 0000000..7e350d6
--- /dev/null
+++ b/libc/arch-arm64/syscalls/__fstatfs.S
@@ -0,0 +1,15 @@
+/* Generated by gensyscalls.py. Do not edit. */
+
+#include <private/bionic_asm.h>
+
+ENTRY(__fstatfs)
+    mov     x8, __NR_fstatfs
+    svc     #0
+
+    cmn     x0, #(MAX_ERRNO + 1)
+    cneg    x0, x0, hi
+    b.hi    __set_errno_internal
+
+    ret
+END(__fstatfs)
+.hidden __fstatfs