fs/nfs, swapon: check holes in swapfile

swapon over NFS does not go through generic_swapfile_activate
code path when setting up extents. This makes holes in NFS
swapfiles possible which is not expected for swapon.

Signed-off-by: Murphy Zhou <jencce.kernel@gmail.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>