Convert zio_buf_alloc() consumers

In multiple cases zio_buf_alloc() was used instead of kmem_alloc()
or vmem_alloc().  This was often done because the allocations
could be large and it was easy to use zfs_buf_alloc() for them.

But this isn't ideal for allocations which are small or short
lived.  In these cases it is better to use kmem_alloc() or
vmem_alloc().  If possible we want to avoid the case where
we have slabs allocated for kmem caches which are rarely used.

Note for small allocations vmem_alloc() will be internally
converted to kmem_alloc().  Therefore as long as large
allocations are infrequent and short lived the penalty for
using vmem_alloc() is small.

Reviewed-by: Chunwei Chen <david.chen@osnexus.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #5409

module/zfs/zap_micro.c

@@ -584,7 +584,7 @@ mzap_upgrade(zap_t **zapp, void *tag, dmu_tx_t *tx, zap_flags_t flags)
 	ASSERT(RW_WRITE_HELD(&zap->zap_rwlock));
 
 	sz = zap->zap_dbuf->db_size;
-	mzp = zio_buf_alloc(sz);
+	mzp = vmem_alloc(sz, KM_SLEEP);
 	bcopy(zap->zap_dbuf->db_data, mzp, sz);
 	nchunks = zap->zap_m.zap_num_chunks;
 
@@ -592,7 +592,7 @@ mzap_upgrade(zap_t **zapp, void *tag, dmu_tx_t *tx, zap_flags_t flags)
 		err = dmu_object_set_blocksize(zap->zap_objset, zap->zap_object,
 		    1ULL << fzap_default_block_shift, 0, tx);
 		if (err) {
-			zio_buf_free(mzp, sz);
+			vmem_free(mzp, sz);
 			return (err);
 		}
 	}
@@ -619,7 +619,7 @@ mzap_upgrade(zap_t **zapp, void *tag, dmu_tx_t *tx, zap_flags_t flags)
 		if (err)
 			break;
 	}
-	zio_buf_free(mzp, sz);
+	vmem_free(mzp, sz);
 	*zapp = zap;
 	return (err);
 }