sheepdog: optimize reloading of inode

Current sheepdog driver reloads inode when in both cases of snapshot
and invalidateion with iSCSI multipath. In the first case, reloading
whole inode isn't required. This patch reduces the overhead produced
by whole reloading in the case of snapshot.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>