Renamed lfsr_attr_t -> lfsr_rattr_t

To avoid the obvious conflict with lfs_attr. Unlike lfsr_rattr_t,
lfs_attr is user facing, so it gets priority.

This name may change in the future if something better comes up, but in
the meantime we need to change the name to _something_.

Is this the reason Linux/BSD/etc call these xattrs?

(Note littlefs's attrs are much more limited than xattrs. We should
_not_ call these xattrs in case we want to add true xattrs in the
future.)