Annotate unused parameters on inline definitions as such

• libspl: umem: These are obviously and intentionally unused; annotate them as such to appease -Wunused-parameter builds that include this header.

• sys/dmu.h: In this case, clear_on_evict_dbufp is only used for ZFS_DEBUG builds, so annotate it as __maybe_unused to appease -Wunused-parameter.

Reviewed-By: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Kyle Evans <kevans@FreeBSD.org>
Closes #10606