allow toolbox block_device:dir search

needed to get to the swap device.

Addresses the following denial:

  avc:  denied  { search } for  pid=149 comm="mkswap" name="block" dev="tmpfs" ino=9947 scontext=u:r:toolbox:s0 tcontext=u:object_r:block_device:s0 tclass=dir permissive=0

Change-Id: I0c897540f1c7950738622a013121a050a1f32b2f