spufs: fix a leak on spufs_new_file() failure

It's called from spufs_fill_dir(), and caller of that will do
spufs_rmdir() in case of failure.  That does remove everything
we'd managed to create, but... the problem dentry is still
negative.  IOW, it needs to be explicitly dropped.

Fixes: 3f51dd91 "[PATCH] spufs: fix spufs_fill_dir error path"
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>