spufs: fix a crash in spufs_create_root()

The spu_fs_context was not set in fc->fs_private, this caused a crash
when accessing ctx->mode in spufs_create_root().

Fixes: d2e0981c ("vfs: Convert spufs to use the new mount API")
Signed-off-by: Emmanuel Nicolet <emmanuel.nicolet@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20191008141342.GA266797@gmail.com