fuse: allow idmapped mounts

Now we have everything in place and we can allow idmapped mounts
by setting the FS_ALLOW_IDMAP flag. Notice that real availability
of idmapped mounts will depend on the fuse daemon. Fuse daemon
have to set FUSE_ALLOW_IDMAP flag in the FUSE_INIT reply.

To discuss:
- we enable idmapped mounts support only if "default_permissions" mode is
enabled, because otherwise we would need to deal with UID/GID mappings in
the userspace side OR provide the userspace with idmapped
req->in.h.uid/req->in.h.gid values which is not something that we probably
want to. Idmapped mounts philosophy is not about faking caller uid/gid.

Some extra links and examples:

- libfuse support
https://github.com/mihalicyn/libfuse/commits/idmap_support

- fuse-overlayfs support:
https://github.com/mihalicyn/fuse-overlayfs/commits/idmap_support

- cephfs-fuse conversion example
https://github.com/mihalicyn/ceph/commits/fuse_idmap

- glusterfs conversion example
https://github.com/mihalicyn/glusterfs/commits/fuse_idmap

Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

include/uapi/linux/fuse.h

@@ -217,6 +217,9 @@
  *  - add backing_id to fuse_open_out, add FOPEN_PASSTHROUGH open flag
  *  - add FUSE_NO_EXPORT_SUPPORT init flag
  *  - add FUSE_NOTIFY_RESEND, add FUSE_HAS_RESEND init flag
+ *
+ *  7.41
+ *  - add FUSE_ALLOW_IDMAP
  */
 
 #ifndef _LINUX_FUSE_H
@@ -252,7 +255,7 @@
 #define FUSE_KERNEL_VERSION 7
 
 /** Minor version number of this interface */
-#define FUSE_KERNEL_MINOR_VERSION 40
+#define FUSE_KERNEL_MINOR_VERSION 41
 
 /** The node ID of the root inode */
 #define FUSE_ROOT_ID 1
@@ -421,6 +424,7 @@ struct fuse_file_lock {
  * FUSE_NO_EXPORT_SUPPORT: explicitly disable export support
  * FUSE_HAS_RESEND: kernel supports resending pending requests, and the high bit
  *		    of the request ID indicates resend requests
+ * FUSE_ALLOW_IDMAP: allow creation of idmapped mounts
  */
 #define FUSE_ASYNC_READ		(1 << 0)
 #define FUSE_POSIX_LOCKS	(1 << 1)
@@ -466,6 +470,7 @@ struct fuse_file_lock {
 
 /* Obsolete alias for FUSE_DIRECT_IO_ALLOW_MMAP */
 #define FUSE_DIRECT_IO_RELAX	FUSE_DIRECT_IO_ALLOW_MMAP
+#define FUSE_ALLOW_IDMAP	(1ULL << 40)
 
 /**
  * CUSE INIT request/reply flags
@@ -984,6 +989,19 @@ struct fuse_fallocate_in {
  */
 #define FUSE_UNIQUE_RESEND (1ULL << 63)
 
+/**
+ * This value will be set by the kernel to
+ * (struct fuse_in_header).{uid,gid} fields in
+ * case when:
+ * - fuse daemon enabled FUSE_ALLOW_IDMAP
+ * - idmapping information is not available and uid/gid
+ *   can not be mapped in accordance with an idmapping.
+ *
+ * Note: an idmapping information always available
+ * for inode creation operations like:
+ * FUSE_MKNOD, FUSE_SYMLINK, FUSE_MKDIR, FUSE_TMPFILE,
+ * FUSE_CREATE and FUSE_RENAME2 (with RENAME_WHITEOUT).
+ */
 #define FUSE_INVALID_UIDGID ((uint32_t)(-1))
 
 struct fuse_in_header {