mds: track which map batch_op is in-use in MDRequest

mds: track which map batch_op is in-use in MDRequest

batch_op is in either CInode::batch_ops or CDentry::batch_ops. Current
code gets the map by dereferencing mdr->in[0] or mdr->dn[0].back().
The problem is that MDCache::path_traverse() may clear mdr->in[0] and
mdr->dn[0]. batch_op leaks when mds kills a request whose mdr->in[0] or
mdr->dn[0] were cleared.

Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
(cherry picked from commit 7768d2c07d66ac015b2e235d38b7ac3616c47c43)

Conflicts:
	src/mds/Mutation.h
	  - octopus does not use the specific ns, use the original one