nfsd: nfsd_file_acquire_local no longer returns GC'd nfsd_file

Now that LOCALIO no longer leans on NFSD's filecache for caching open
files (and instead uses NFS client-side open nfsd_file caching) there
is no need to use NFSD filecache's GC feature.  Avoiding GC will speed
up nfsd_file initial opens.

Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Acked-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>