os/bluestore: manage vector from ExtentList

os/bluestore: manage vector from ExtentList

ExtentList was previous relying the caller to preallocate/size the
vector to be large enough for the worst case allocation of extents,
and keeping it's own manual count of the extent list size.  Instead,
manage that from ExtentList, and remove the preallocation from the
callers.

Fixes: http://tracker.ceph.com/issues/18573
Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit 65502891dd66569c8d99384e63b68decdab1c8e2)