crimson/osd: make load_obc(obc, md_ref) return void

crimson/osd: make load_obc(obc, md_ref) return void

load_obc() taking an already-resolved loaded_object_md_t::ref is
synchronous, because it just populates obc state, it does yield.
Returning an errorated future was unnecessary and caused a
-Wunused-result warning at its only call site:
ECRecoveryBackend::maybe_load_obc().

In this change, we change it to return void and deduplicate the OBC
population logic: the private async overload (taking future<md_ref>)
now validates ssc and returns object_corrupted on failure.

This silences the warning, and simpler this way. The async error
propagation is preserved.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>