kerneldump: fix incorrect SETSIZE to BIT_COPY_STORE_REL when livedump

Also replace malloc/free with BITSET_ALLOC/BITSET_FREE macros.

Sponsored by: Juniper Networks, Inc.
MFC after: 1 week
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D47708

(cherry picked from commit 9ad8116b54616a5e8cbcdd2e1432893dce0c608c)