bluestore/bdev: initialize size when creating object.

bluestore/bdev: initialize size when creating object.

The other partss of the struct are initialized by their
ctors. Only for unint_64 there is no ctor.

Otherwise ceph-dencoder tests will fail in comparing
the exported output.
[~/master36] wjw@cephdev.digiware.nl> build/bin/ceph-dencoder type bluestore_bdev_label_t select_test 1 encode export /tmp/typ-yFISvjvgj
[~/master36] wjw@cephdev.digiware.nl> hexdump -C !$
hexdump -C /tmp/typ-yFISvjvgj
00000000  62 6c 75 65 73 74 6f 72  65 20 62 6c 6f 63 6b 20 |bluestore block |
00000010  64 65 76 69 63 65 0a 30  30 30 30 30 30 30 30 2d |device.00000000-|
00000020  30 30 30 30 2d 30 30 30  30 2d 30 30 30 30 2d 30 |0000-0000-0000-0|
00000030  30 30 30 30 30 30 30 30  30 30 30 0a 02 01 28 00 |00000000000...(.|
00000040  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00 |................|
00000050  00 00 70 74 03 04 00 00  00 00 00 00 00 00 00 00 |..pt............|
00000060  00 00 00 00 00 00 00 00  00 00 |..........|
0000006a

[~/master36] wjw@cephdev.digiware.nl> build/bin/ceph-dencoder type bluestore_bdev_label_t select_test 1 encode decode encode export /tmp/typ-MjWXdCpzJ
[~/master36] wjw@cephdev.digiware.nl> hexdump -C !$
hexdump -C /tmp/typ-MjWXdCpzJ
00000000  62 6c 75 65 73 74 6f 72  65 20 62 6c 6f 63 6b 20 |bluestore block |
00000010  64 65 76 69 63 65 0a 30  30 30 30 30 30 30 30 2d |device.00000000-|
00000020  30 30 30 30 2d 30 30 30  30 2d 30 30 30 30 2d 30 |0000-0000-0000-0|
00000030  30 30 30 30 30 30 30 30  30 30 30 0a 02 01 28 00 |00000000000...(.|
00000040  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00 |................|
00000050  00 00 73 64 00 00 00 00  00 00 00 00 00 00 00 00 |..sd............|
00000060  00 00 00 00 00 00 00 00  00 00 |..........|
0000006a

Signed-off-by: Willem Jan Withagen <wjw@digiware.nl>
(cherry picked from commit d411ee26fb2f6cbe610f8bbc81b777cf28d839c2)