dm space map disk: cache a small number of index entries

The disk space map stores it's index entries in a btree, these are
accessed very frequently, so having a few cached makes a big difference
to performance.

With this change provisioning a new block takes roughly 20% less cpu.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>