crimson/osd: add PG::with_head_obc()

crimson/osd: add PG::with_head_obc()

this method replicates `PG::get_or_load_head_obc()`. but uses a different
way to ensure that the "lock" on obc is always released even if the
called func throws. it always guard the called func with a
`with_lock()`, so `lock.unlock()` is always called. the plan is to
replace `PG::get_or_load_head_obc()` with `PG::with_head_obc()` in
the following changes piecemeal.

Signed-off-by: Kefu Chai <kchai@redhat.com>