Return NOTTY for all btrfs ioctl calls

mergerfs tries to pass through ioctl calls but due to the complexity
of btrfs calls their is risk of segfaults. In the future it may be
required to whitelist all supported ioctl calls.

src/fuse_ioctl.cpp

@@ -44,6 +44,9 @@ typedef char IOCTL_BUF[4096];
 #define IOCTL_APP_TYPE  0xDF
 #define IOCTL_FILE_INFO _IOWR(IOCTL_APP_TYPE,0,IOCTL_BUF)
 
+// From linux/btrfs.h
+#define BTRFS_IOCTL_MAGIC 0x94
+
 #ifndef FS_IOC_GETFLAGS
 # define FS_IOC_GETFLAGS _IOR('f',1,long)
 #endif
@@ -314,6 +317,13 @@ namespace l
     return (_IOC_TYPE(cmd_) == IOCTL_APP_TYPE);
   }
 
+  static
+  bool
+  is_btrfs_ioctl_cmd(const unsigned long cmd_)
+  {
+    return (_IOC_TYPE(cmd_) == BTRFS_IOCTL_MAGIC);
+  }
+
   static
   int
   ioctl_custom(const fuse_file_info_t *ffi_,
@@ -340,6 +350,8 @@ namespace FUSE
         void                   *data_,
         uint32_t               *out_bufsz_)
   {
+    if(l::is_btrfs_ioctl_cmd(cmd_))
+      return -ENOTTY;
     if(l::is_mergerfs_ioctl_cmd(cmd_))
       return l::ioctl_custom(ffi_,cmd_,data_);