Allow fsck to search /dev/block.

Addresses denials such as:
avc:  denied  { search } for  pid=143 comm="e2fsck" name="block" dev="tmpfs" ino=5987 scontext=u:r:fsck:s0 tcontext=u:object_r:block_device:s0 tclass=dir

Change-Id: Ieb72fc5e28146530c2f3b235ce74f2f397e49c56
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>