Destroy bdi on error in fuse_fill_super().

This was an omission from commit 26c36791
"fuse: destroy bdi on umount", which moved the bdi_destroy() call from
fuse_conn_put() to fuse_put_super().
Signed-off-by: NMiklos Szeredi <mszeredi@suse.cz>
CC: stable@kernel.org