Remove ASSERT() in l2arc_apply_transforms()

The ASSERT was erroneously copied from the next section of code.
The buffer's size should be expanded from "psize" to "asize"
if necessary.

Reviewed-by: Tom Caputi <tcaputi@datto.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tim Chase <tim@chase2k.com>
Closes #7375