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>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@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>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
sheepdog: change a method for detecting invalidation of inode

Current method of detecting invalidation of inode can produce bunch of
error messages if a first object of VDI doesn't exist. This patch
updates the method: reading first one byte of inode object. It can
reduce needless and confusing error messages.

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