dm exception store: split set_chunk_size

commit 2defcc3f upstream.

Break the function set_chunk_size to two functions in preparation for
the fix in the following patch.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>