Corrected edge case in uncompressed ARC->L2ARC handling

I genuinely don't know why this didn't come up before,
but adding the LZ4 early abort pointed out this flaw,
in which we're allocating a buffer of one size, and
then telling the compressor that we're handing it buffers
of a different size, which may be Very Different - say,
allocating 512b and then telling it the inputs are 128k.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Amanakis <gamanakis@gmail.com>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #13375