Reinstate returning EOVERFLOW from stats_v1_blob_clone()

[lstewart]

Summary

a0993376ec5f (from D43179) subtly changed stats_v1_blob_clone() to stop returning EOVERFLOW in the case where the user buffer is not large enough to receive the entire statsblob. This results in any consumers which are implemented to retry on receiving EOVERFLOW to instead give up after receiving an empty statsblob header.

Fix by latching any errors recorded prior to copyout.

Obtained from: Netflix, Inc.

[markj]

Thank you. Please add a Fixes: a0993376ec5f ("stats: Check for errors from copyout()") tag to the commit log message.