test/bufferlist.cc: encode/decode int64_t instead of long

test/bufferlist.cc: encode/decode int64_t instead of long

use `int64_t` instead of `long`, as on LP64 systems, `long` are 64bit.
and LP64 is quite popular nowadays.

quote from Alexandre's mail:

> encode/decode don't handle long at all, they deal with u?int{32,64}_t
> and uint64_t.  When int64_t happens to be int or long, this works, but
> when int64_t is long long, then long ends up uncovered

Signed-off-by: Alexandre Oliva <oliva@gnu.org>