Add missing arc_free_cksum() to arc_release()

Add missing arc_free_cksum() to arc_release()

The arc layer tracks checksums of its data in the arc header
so that it can ensure that buffers haven't changed when they're
not supposed to. This checksum is only maintained while there
is an uncompressed buffer still attached to the header.
Unfortunately there is a missing call to arc_free_cksum() in
arc_release() that can trigger ASSERTs. This has not been a
common issue because the checksums are only maintained for
debug builds and triggering the bug requires writing a block
(and therefore calling arc_release()) while a compressed buffer
is still being used on a debug build. This simply corrects the
issue.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes #6105