cifs: when renaming don't try to unlink negative dentry

When attempting to rename a file on a read-only share, the kernel can
call cifs_unlink on a negative dentry, which causes an oops. Only try
to unlink the file if it's a positive dentry.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Tested-by: Shirish Pargaonkar <shirishp@us.ibm.com>
CC: Stable <stable@vger.kernel.org>
Signed-off-by: Steve French <sfrench@us.ibm.com>