loader/zfs: Fix to actually return the last error

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Summary

The last fix, to try to return the last error, really returns the first
return code after the last error, which could be zero. Instead, return
the last error. Also, change rc to err to make it visually distinct from
rv, which is the cause of my error in e54bb0ad8058.

Reported by: Bill Sommerfeld
Fixes: e54bb0ad8058
Sponsored by: Netflix

[imp]

stand/libsa/zfs/zfsimpl.c

1699

I'm not sure why the original code didn't return error here... we're returning the last error after trying everything, which likely is why...

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525e6d6c890f6aee898ac70347e5bb49da6638a1 landed this, or something substantially similar.