spufs: fix a leak in spufs_create_context()

Leak fixes back in 2008 missed one case - if we are trying to set affinity
and spufs_mkdir() fails, we need to drop the reference to neighbor.

Fixes: 58119068 "[POWERPC] spufs: Fix memory leak on SPU affinity"
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>