ZAP_BLOCK_SIZE became unused in the preemptible-mmu_gather work ("mm:
Remove i_mmap_lock lockbreak").  So zap it.

Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>