client: fix simultaneous readdirs race

client: fix simultaneous readdirs race

Current readdir code uses list to track the order of the dentries
in readdir replies.  When handling a readdir reply, it pushes the
resulting dentries to the back of directory's dentry_list. After
readdir finishes, the dentry_list reflects how MDS sorts dentries.

This method is racy when there are simultaneous readdirs. The fix
is use vector instead of list to trace how dentries are sorted in
its parent directory. As long as shared_gen doesn't change, each
dentry is at fixed position of the vector. So cocurrent readdirs
do not affect each other.

Fixes: http://tracker.ceph.com/issues/15508
Signed-off-by: Yan, Zheng <zyan@redhat.com>