NFS Exclusive open not supported bug

When trying to open a file with the O_EXCL flag over NFS on a server that does
not support exclusive mode, the file does not open.  The reason,
rpc_call_sync returns a errno number, and not the nfs error number.  I fixed
it by changing the status check in nfs3proc.c.  Either this is how it should
be fixed, or rpc_call_sync should be fixed to return the NFS error.

Signed-off-by: Andy Ryan <genanr@allantgroup.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>