fuse: rename FUSE_WRITE_KILL_PRIV to FUSE_WRITE_KILL_SUIDGID

Kernel has:
ATTR_KILL_PRIV -> clear "security.capability"
ATTR_KILL_SUID -> clear S_ISUID
ATTR_KILL_SGID -> clear S_ISGID if executable

Fuse has:
FUSE_WRITE_KILL_PRIV -> clear S_ISUID and S_ISGID if executable

So FUSE_WRITE_KILL_PRIV implies the complement of ATTR_KILL_PRIV, which is
somewhat confusing.  Also PRIV implies all privileges, including
"security.capability".

Change the name to FUSE_WRITE_KILL_SUIDGID and make FUSE_WRITE_KILL_PRIV an
alias to perserve API compatibility
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/file.c

@@ -1451,7 +1451,7 @@ ssize_t fuse_direct_io(struct fuse_io_priv *io, struct iov_iter *iter,
 
 		if (write) {
 			if (!capable(CAP_FSETID))
-				ia->write.in.write_flags |= FUSE_WRITE_KILL_PRIV;
+				ia->write.in.write_flags |= FUSE_WRITE_KILL_SUIDGID;
 
 			nres = fuse_send_write(ia, pos, nbytes, owner);
 		} else {

include/uapi/linux/fuse.h

@@ -177,7 +177,7 @@
  *  - add flags to fuse_attr, add FUSE_ATTR_SUBMOUNT, add FUSE_SUBMOUNTS
  *
  *  7.33
- *  - add FUSE_HANDLE_KILLPRIV_V2
+ *  - add FUSE_HANDLE_KILLPRIV_V2, FUSE_WRITE_KILL_SUIDGID
  */
 
 #ifndef _LINUX_FUSE_H
@@ -387,11 +387,14 @@ struct fuse_file_lock {
  *
  * FUSE_WRITE_CACHE: delayed write from page cache, file handle is guessed
  * FUSE_WRITE_LOCKOWNER: lock_owner field is valid
- * FUSE_WRITE_KILL_PRIV: kill suid and sgid bits
+ * FUSE_WRITE_KILL_SUIDGID: kill suid and sgid bits
  */
 #define FUSE_WRITE_CACHE	(1 << 0)
 #define FUSE_WRITE_LOCKOWNER	(1 << 1)
-#define FUSE_WRITE_KILL_PRIV	(1 << 2)
+#define FUSE_WRITE_KILL_SUIDGID (1 << 2)
+
+/* Obsolete alias; this flag implies killing suid/sgid only. */
+#define FUSE_WRITE_KILL_PRIV	FUSE_WRITE_KILL_SUIDGID
 
 /**
  * Read flags