[CIFS] use common code for turning off ATTR_READONLY in cifs_unlink

We already have a cifs_set_file_info function that can flip DOS
attribute bits. Have cifs_unlink call it to handle turning ATTR_HIDDEN
on and ATTR_READONLY off when an unlink attempt returns -EACCES.

This also removes a level of indentation from cifs_unlink.

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