rgw/reshard: bucket reshard may race with bucket index update.

rgw/reshard: bucket reshard may race with bucket index update.
guard_reshard() serves as a wrapper to protect bucket index operations
against concurrent resharding. but it only handles -ERR_BUSY_RESHARDING
if bucket index update is already in progress with bucket shard
assigned, and a concurrent resharding operation deletes this old shard
object, there may be a window of operations targeting the old shards
failing with -ENOENT. this error is not caught anywhere in the calling
functions. as a result we could end up with objects in an inconsistent state.

Signed-off-by: Shilpa Jagannath <smanjara@redhat.com>