bcachefs_effective.* xattrs show the options inherited from parent
directories (as well as explicitly set); this namespace is not for
setting bcachefs options.

Change the .set() handler to a noop so that if e.g. rsync is copying
xattrs it'll do the right thing, and only copy xattrs in the bcachefs.*
namespace. We don't want to return an error, because that will cause
rsync to bail out or get spammy.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>