swapon(2) requires write access to the underlying block device.
Allow it.

Addresses the following denial:

   avc:  denied  { write } for  pid=1 comm="init" name="zram0" dev="tmpfs" ino=6267 scontext=u:r:init:s0 tcontext=u:object_r:swap_block_device:s0 tclass=blk_file permissive=0

Change-Id: Id1a4f51038d0b6ce7351294698a0ff146d6e4643