pnfsblock: clean up _add_entry

It is wrong to kmalloc in _add_entry() as it is inside
spinlock. memory should be already allocated _add_entry() is called.

Signed-off-by: Peng Tao <peng_tao@emc.com>
Signed-off-by: Benny Halevy <bhalevy@tonian.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>