init: allow init to restorecon on block devices and their symlinks

For early mount we end up creating the device nodes for partitions
under /dev/block before selinux is initialized. Which means, that
restorecon_recursive on /dev/block will have to relabel these nodes
and their symlinks.

This change adds the rule to allow init do the same.

b/27805372

Test: boot marlin / sailfish with early mount device node creation
but mount partitions using the default 'mountall' without any selinux
denials.

Change-Id: Ib9335f3f961d485d2120a175dbdbf85d6f70b160
Signed-off-by: Sandeep Patil <sspatil@google.com>