client: introduce a new flag indicating if dentries in directory are sorted

When creating a file, Client::insert_dentry_inode() set the dentry's offset
based on directory's max offset. The offset does not reflect the real
postion of the dentry in directory. Later readdir reply from real postion
of the dentry in directory. Later readdir reply from MDS may change the
dentry's position/offset. This inconsistency can cause missing/duplicate
entries in readdir result if readdir is partly satisfied by dcache_readdir().

The fix is introduce a new flag indicating if dentries in directory are
sorted. We use _readdir_cache_cb() to handle readdir only when the flag is
set.  After creating/rename file, we clear the flag.

Fixes: #6613
Signed-off-by: Yan, Zheng <zyan@redhat.com>
client: preserve ordering of readdir result in cache

Preserve ordering of readdir result in a list, so that the result of cached
readdir is consistant with uncached readdir.

As a side effect, this commit also removes the code that removes stale dentries.
This is OK because stale dentries does not have valid lease, they will be
filter out by the shared gen check in Client::_readdir_cache_cb()

Signed-off-by: Yan, Zheng <zyan@redhat.com>