dm exception store: split set_chunk_size

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

Cc: stable@kernel.org
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

diff --cc drivers/md/dm-exception-store.c
Simple merge

diff --cc drivers/md/dm-exception-store.h
Simple merge