A previous change moved the GETATTR RPC for open() calls that hit in the

A previous change moved the GETATTR RPC for open() calls that hit in the
name cache up into nfs_lookup() instead of nfs_open().  Continue this
trend by flushing the attribute cache for leaf nodes in nfs_lookup() during
an open() if we do a LOOKUP RPC.  For NFSv3 this should generally be a NOP
as the attributes are flushed before fetching the post-op attributes from
the LOOKUP RPC which most (all?) NFSv3 servers provide, so the post-op
attributes should populate the cache.

Now all NFS open() calls will always clear the cached attributes during the
nfs_lookup() prior to nfs_open() in the !NMODIFIED case to provide CTOC.
As a result, we can remove the conditional flushing of the attribute
cache from nfs_open().

Reviewed by:	rmacklem, bde
MFC after:	2 weeks