iomap: don't allow holes in swapfiles

generic_swapfile_activate() doesn't allow holes, so we should be
consistent here. This is also a bit safer: if the user creates a
swapfile with, say, truncate -s $SIZE followed by mkswap, they should
really get an error and not much less swap space than they expected.
swapon(8) will error out before calling swapon(2) if the file has holes,
anyways.

Fixes: 9d93388b0afe ("iomap: add a swapfile activation function")
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Omar Sandoval <osandov@fb.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>