beginning of fs side work:
	* new helpers - old_valid_dev(), old_encode_dev() and old_decode_dev()
	* old_valid_dev() checks if dev_t value is OK for old filesystems
(i.e. both major and minor are below 256).
	* old_valid_dev() calls are added in ->mknod() instances that care
about dev_t values (disk-backed and network ones).
	* old_encode_dev() and old_decode_dev() convert dev_t -> u16
and u16 -> dev_t resp; currently these are no-ops, places that use current
formar (minor in bits 0--7, major in bits 8--15) will switch to these before
we widen dev_t.