More gracefully handle stale file handles and attributes when opening a

More gracefully handle stale file handles and attributes when opening a
file via NFS.  Specifically, to satisfy close-to-open-consistency, the NFS
client always performs at least one RPC on a file during an open(2) to see
if the file has changed.  Normally this RPC is an ACCESS or GETATTR RPC
that is forced by flushing a file's attribute cache during nfs_open() and
then requesting new attributes.  However, if the file is noticed to be
stale during nfs_open(), the only recourse is to fail the open(2) call
with ESTALE.  On the other hand, if the ACCESS or GETATTR RPC is sent
during nfs_lookup(), then the NFS client can fall back to a LOOKUP RPC to
obtain the new file handle in the case that a file has been replaced.

This change causes the NFS client to flush the attribute cache during
nfs_lookup() when validating a name cache hit if the attributes fetched
during nfs_lookup() can be reused in nfs_open().  This allows the client
to open a replaced file via the new file handle the first time that it
notices a replaced file rather than failing with ESTALE in some cases.

Reviewed by:	rmacklem, bde
Reviewed by:	mohans (older version)
MFC after:	1 week