NFS: Fix the mapping of the NFSERR_SERVERFAULT error

It was recently pointed out that the NFSERR_SERVERFAULT error, which is
designed to inform the user of a serious internal error on the server, was
being mapped to an error value that is internal to the kernel.

This patch maps it to the error EREMOTEIO, which is exported to userland
through errno.h.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Cc: stable@kernel.org