cifs: eliminate redundant xdev check in cifs_rename

The VFS always checks that the source and target of a rename are on the
same vfsmount, and hence have the same superblock. So, this check is
redundant. Remove it and simplify the error handling.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>