crimson/os/seastore/cache: not all exist_clean extents need deepcopying

before mutation

At present, only OBJECT_DATA_BLOCK and LOG_NODE extents share the
content ptr when being duplicated for writes and OBJECT_DATA_BLOCK
extents' mutations don't affect the original content, so only LOG_NODE
extents might need deepcopying when turning from EXIST_CLEAN to
EXIST_MUTATION_PENDING.

Signed-off-by: Xuehan Xu <xuxuehan@qianxin.com>