Cleanup some ESTALE issues on the client when files are replaced on

Cleanup some ESTALE issues on the client when files are replaced on
an NFS server.  Even though the attribute cache has expired DragonFly
still maintains a vnode in the namecache.  If a file is replaced on the
server the vnode's file handle will become invalid.

Force re-resolution of the namecache entry rather then replacing the vnode's
file handle, so programs with open descriptors to the dead file continue to
get a proper error return while lookups succeed in finding the new version
of the file.

In this patch ESTALE is checked in strategic places:  stat(), access(), and
open().  It is an imperfect solution at the moment but it seems to work pretty
well.  This should bring NFS client operations back up to FreeBSD-4.x
standards.