CIFS: Fix directory rename error

CIFS: Fix directory rename error

CIFS servers process nlink counts differently for files and directories.
In cifs_rename() if we the request fails on the existing target, we
try to remove it through cifs_unlink() but this is not what we want
to do for directories. As the result the following sequence of commands

mkdir {1,2}; mv -T 1 2; rmdir {1,2}; mkdir {1,2}; echo foo > 2/bar

and XFS test generic/023 fail with -ENOENT error. That's why the second
mkdir reuses the existing inode (target inode of the mv -T command) with
S_DEAD flag.

Fix this by checking whether the target is directory or not and
calling cifs_rmdir() rather than cifs_unlink() for directories.

Cc: <[email protected]>
Signed-off-by: Pavel Shilovsky <[email protected]>
Signed-off-by: Steve French <[email protected]>