sheepdog: protocol update for revivng backward compatibility

This update is just for updating protocol of sheepdog. Recent
discussion in the sheepdog community requires it.

Singed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
sheepdog: refresh invalidated inode in update_inode() path

This patch also adds a string representation of new error code
SD_RES_INODE_INVALIDATED.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
sheepdog: set flag for indicating requests are from tgt

It is required for iSCSI multipath with sheepdog.

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