crimson/osd: Introduce with_head_and_clone_obc()

crimson/osd: Introduce with_head_and_clone_obc()

In continuation to 7ca2690be956a36f61c7729946b94ccd970dd9c7:
Now that the head ref is no longer a member of obc, we need a new
substitute way to get the head when needed.

When loading a clone object, the head object is loaded
first (See with_clone_obc). Therefore we can make use of this design
to move the loaded head forward to the relevant func (See with_head_and_clone_obc).
Usually, we wouldn't need to make use of both the head and the clone obc in the
same function. However, SnapTrimObjSubEvent::remove_or_update is an abnormal usage.

Note: We want to avoid holding any unneeded references to obcs
to allow the obc_registery to evict no longer valid obc.
Therefore, with_obc() which references only a single obc is the
preferred entry point for loading obcs.

Signed-off-by: Matan Breizman <mbreizma@redhat.com>