cifs: flush data on any setattr

We already flush all the dirty pages for an inode before doing
ATTR_SIZE and ATTR_MTIME changes. There's another problem though -- if
we change the mode so that the file becomes read-only then we may not
be able to write data to it after a reconnect.

Fix this by just going back to flushing all the dirty data on any
setattr call. There are probably some cases that can be optimized out,
but I'm not sure they're worthwhile and we need to consider them more
carefully to make sure that we don't cause regressions if we have
to reconnect before writeback occurs.

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