libfs: set SB_I_NOEXEC and SB_I_NODEV by default in init_pseudo()

Since commit 1e7ab6f678 ("anon_inode: rework assertions"),
path_noexec() warns when an anonymous-inode file is mmap'd from a
superblock that has not set SB_I_NOEXEC. dma-buf backs its files this
way and never set the flag, so mmap of any exported buffer trips the
warning on a CONFIG_DEBUG_VFS=y kernel:

  WARNING: CPU: 11 PID: 121813 at fs/exec.c:118 path_noexec+0x47/0x50
   do_mmap+0x2b5/0x680
   vm_mmap_pgoff+0x129/0x210
   ksys_mmap_pgoff+0x177/0x240
   __x64_sys_mmap+0x33/0x70

init_pseudo() sets up internal SB_NOUSER mounts that are never
path-reachable. Set both flags here so every pseudo filesystem gets
them by default instead of each caller setting them.

SB_I_NODEV is inert for unreachable mounts. SB_I_NOEXEC has one
visible effect: an executable mapping of a pseudo-fs fd, such as a
dma-buf, now fails with -EPERM, which is the invariant the assertion
enforces. No in-tree caller maps these executable.

Reproduce on CONFIG_DEBUG_VFS=y:

  make -C tools/testing/selftests/dmabuf-heaps
  sudo ./tools/testing/selftests/dmabuf-heaps/dmabuf-heap -t system

Fixes: 1e7ab6f678 ("anon_inode: rework assertions")
Suggested-by: Christoph Hellwig <hch@infradead.org>
Cc: stable@vger.kernel.org
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Link: https://patch.msgid.link/20260604025315.245910-2-jhubbard@nvidia.com
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

fs/libfs.c

@@ -736,6 +736,7 @@ struct pseudo_fs_context *init_pseudo(struct fs_context *fc,
 		fc->fs_private = ctx;
 		fc->ops = &pseudo_fs_context_ops;
 		fc->sb_flags |= SB_NOUSER;
+		fc->s_iflags |= SB_I_NOEXEC | SB_I_NODEV;
 		fc->global = true;
 	}
 	return ctx;