Fix memory leaks in dmu_send()/dmu_send_obj()

If we encounter an EXDEV error when using the redacted snapshots
feature, the memory used by dspp.fromredactsnaps is leaked.

Clang's static analyzer caught this during an experiment in which I had
annotated various headers in an attempt to improve the results of static
analysis.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #13973