crimson/os/seastore: do "copy_on_write" if the to-be-modified object

crimson/os/seastore: do "copy_on_write" if the to-be-modified object
needs it

At present, only clone objects may need COW, as HEAD objects won't be
sharing any direct lba mapping with other objects in non-recovery
scenarios.

Although the HEAD object may share its direct mappings with the temp
object that's going to be recovered and replace it, it won't be
accepting any modifications at that time.

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