When failing to allocate the sessions memory we should make sure
the req1 and req2 and the sessions get put. And also in case the
max_sessions decreased so when kreallocate the new memory some
sessions maybe missed being put.

And if the max_sessions is 0 krealloc will return ZERO_SIZE_PTR,
which will lead to a distinct access fault.

URL: https://tracker.ceph.com/issues/53819
Fixes: e1a4541e ("ceph: flush the mdlog before waiting on unsafe reqs")
Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Venky Shankar <vshankar@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>