jbd2: use kmem_cache_zalloc instead of kmem_cache_alloc/memset

The jbd2_alloc_handle() function is only called by new_handle().  So
this commit uses kmem_cache_zalloc() instead of
kmem_cache_alloc()/memset().

Signed-off-by: Zheng Liu <wenqing.lz@taobao.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>