os/filestore: track objects with writes being applied

os/filestore: track objects with writes being applied

Note that this is *slight* overkill in that a *source* object of a clone
will also appear in the applying map, even though it is not being
modified.  Given that those clone operations are normally coupled with
another transaction that does write (which is why we are cloning in the
first place) this should not make any difference.

Signed-off-by: Sage Weil <sage@redhat.com>