mds: try to choose a new batch head in request_clientup()

mds: try to choose a new batch head in request_clientup()

This will happen only for the client requests, not peer requests.
The 'mdr->killed' and 'mdr->dead' will always be set at the same
time when killing the client requests.

Fixes: https://tracker.ceph.com/issues/66124
Signed-off-by: Xiubo Li <xiubli@redhat.com>
(cherry picked from commit 73b266e53632de270af31588da497f7162301fc0)

 Conflicts:
	src/mds/MDCache.cc
	src/mds/Server.cc
	Missing commit 99e58f58547("mds: use const qualifier for
	MDRequestRef").