dm-verity: remove useless mempool

v->fec->extra_pool has zero reserved entries, so we can remove it and use
the kernel cache directly.

Signed-off-by: Mikulas Patocka <[e-mail]>
Reviewed-by: Eric Biggers <[e-mail]>

drivers/md/dm-verity-fec.c

···
 		if (fio->bufs[n])
 			continue;
 
-		fio->bufs[n] = mempool_alloc(&v->fec->extra_pool, GFP_NOWAIT);
+		fio->bufs[n] = kmem_cache_alloc(v->fec->cache, GFP_NOWAIT);
 		/* we can manage with even one buffer if necessary */
 		if (unlikely(!fio->bufs[n]))
 			break;
···
 		mempool_free(fio->bufs[n], &f->prealloc_pool);
 
 	fec_for_each_extra_buffer(fio, n)
-		mempool_free(fio->bufs[n], &f->extra_pool);
+		if (fio->bufs[n])
+			kmem_cache_free(f->cache, fio->bufs[n]);
 
 	mempool_free(fio->output, &f->output_pool);
 }
···
 
 	mempool_exit(&f->rs_pool);
 	mempool_exit(&f->prealloc_pool);
-	mempool_exit(&f->extra_pool);
 	mempool_exit(&f->output_pool);
 	kmem_cache_destroy(f->cache);
 
···
 				     f->cache);
 	if (ret) {
 		ti->error = "Cannot allocate FEC buffer prealloc pool";
-		return ret;
-	}
-
-	ret = mempool_init_slab_pool(&f->extra_pool, 0, f->cache);
-	if (ret) {
-		ti->error = "Cannot allocate FEC buffer extra pool";
 		return ret;
 	}
 

drivers/md/dm-verity-fec.h

···
 	unsigned char rsn;	/* N of RS(M, N) */
 	mempool_t rs_pool;	/* mempool for fio->rs */
 	mempool_t prealloc_pool;	/* mempool for preallocated buffers */
-	mempool_t extra_pool;	/* mempool for extra buffers */
 	mempool_t output_pool;	/* mempool for output */
 	struct kmem_cache *cache;	/* cache for buffers */
 	atomic64_t corrected; /* corrected errors */