Restore zero-copy buffers in OSD fast path.

This change restores volatile sharing semantics in the Message
decode path, and also in the OSD write path for FileStore/FileJournal.

This can be verified with a breakpoint set at the clone/COW case in
buffer::ptr::clone_nonsharable(currently buffer.cc:690).

Signed-off-by: Matt Benjamin <matt@cohortfs.com>
Message::set_data cannot preserve constness.

Signed-off-by: Matt Benjamin <matt@cohortfs.com>