client: avoid freeing inode when it contains TX buffer heads

client: avoid freeing inode when it contains TX buffer heads

ObjectCacher::discard_set() prematurely delete TX buffer heads. But
the pending writebacks still pin parent objects of these buffer heads.
Assertion "oset.objects.empty()" gets triggered if inode with pending
writebacks get freed.

Fixes: http://tracker.ceph.com/issues/23837
Signed-off-by: Guan yunfei <yunfei.guan@xtaotech.com>
Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
(cherry picked from commit 8a03757ca0ab493c6c2ea4fa4307e053e8ebc944)