os/bluestore/BlueFS: apply shared_alloc_size to shared device

os/bluestore/BlueFS: apply shared_alloc_size to shared device

Keep an alloc_size vector so that we have this value handy at all times.
Allow bluestore to fetch this value directly instead of looking at the
bluefs_* config options since this encapsulates things a bit better, and
also isn't vulnerable to the config setting changing at runtime.

Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit e8b5a458c306dfe166b09239fc9f57c04fa47ead)