can't get_block(NO_DEV) any more

can't get_block(NO_DEV) any more

	. 'anonymous' cache blocks (retrieved with NO_DEV as dev
	  parameter) were used to implement read()s from holes in
	  inodes that should return zeroes
	. this is an awkward special case in the cache code though
	  and there's a more direct way to implement the same functionality:
	  instead of copying from a new, anonymous, zero block, to
	  the user target buffer, simply sys_safememset the user target
	  buffer directly. as this was the only use of this feature,
	  this is all that's needed to simplify the cache code a little.