romfs: fix returm err while getting inode in fill_super

Getting an inode by romfs_iget may lead to an err in fill_super, and the
err value should be return.

And it should return -ENOMEM instead while d_make_root fails, fix it too.

Signed-off-by: Rui Xiang <rui.xiang@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>