NFS: Return -ETXTBSY when attempting to write to a swapfile

My understanding is that -EBUSY refers to the underlying device, and
that -ETXTBSY is used when attempting to access a file in use by the
kernel (like a swapfile). Changing this return code helps us pass
xfstests generic/569

Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>