fuse: handle killpriv in userspace fs

Only userspace filesystem can do the killing of suid/sgid without races.
So introduce an INIT flag and negotiate support for this.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h
index 27e1736..14ca2f1 100644
--- a/include/uapi/linux/fuse.h
+++ b/include/uapi/linux/fuse.h
@@ -108,6 +108,9 @@
  *
  *  7.25
  *  - add FUSE_PARALLEL_DIROPS
+ *
+ *  7.26
+ *  - add FUSE_HANDLE_KILLPRIV
  */
 
 #ifndef _LINUX_FUSE_H
@@ -143,7 +146,7 @@
 #define FUSE_KERNEL_VERSION 7
 
 /** Minor version number of this interface */
-#define FUSE_KERNEL_MINOR_VERSION 25
+#define FUSE_KERNEL_MINOR_VERSION 26
 
 /** The node ID of the root inode */
 #define FUSE_ROOT_ID 1
@@ -238,6 +241,7 @@
  * FUSE_WRITEBACK_CACHE: use writeback cache for buffered writes
  * FUSE_NO_OPEN_SUPPORT: kernel supports zero-message opens
  * FUSE_PARALLEL_DIROPS: allow parallel lookups and readdir
+ * FUSE_HANDLE_KILLPRIV: fs handles killing suid/sgid/cap on write/chown/trunc
  */
 #define FUSE_ASYNC_READ		(1 << 0)
 #define FUSE_POSIX_LOCKS	(1 << 1)
@@ -258,6 +262,7 @@
 #define FUSE_WRITEBACK_CACHE	(1 << 16)
 #define FUSE_NO_OPEN_SUPPORT	(1 << 17)
 #define FUSE_PARALLEL_DIROPS    (1 << 18)
+#define FUSE_HANDLE_KILLPRIV	(1 << 19)
 
 /**
  * CUSE INIT request/reply flags