arch/sh/kernel/dwarf.c: destroy mempools on cleanup

dwarf_reg_pool and dwarf_frame_pool are not properly destroyed when
cleaning up the dwarf unwinder.  Destroy them with mempool_destroy().

Also mark dwarf_unwinder_cleanup() as __init.

Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>