block: bvec_nr_vecs() returns value for wrong slab

In commit ed996a52c868 ("block: simplify and cleanup bvec pool
handling"), the value of the slab index is incremented by one in
bvec_alloc() after the allocation is done to indicate an index value of
0 does not need to be later freed.

bvec_nr_vecs() was not updated accordingly, and thus returns the wrong
value.  Decrement idx before performing the lookup.

Fixes: ed996a52c868 ("block: simplify and cleanup bvec pool handling")
Signed-off-by: Greg Edwards <gedwards@ddn.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bio.c

@@ -158,7 +158,7 @@ out:
 
 unsigned int bvec_nr_vecs(unsigned short idx)
 {
-	return bvec_slabs[idx].nr_vecs;
+	return bvec_slabs[--idx].nr_vecs;
 }
 
 void bvec_free(mempool_t *pool, struct bio_vec *bv, unsigned int idx)