libxfs: use FALLOC_FL_ZERO_RANGE in libxfs_device_zero

libxfs: use FALLOC_FL_ZERO_RANGE in libxfs_device_zero

I had a request from someone who cared about mkfs speed over
a slower network block device to look into using faster zeroing
methods, particularly for the log, during mkfs.

Using FALLOC_FL_ZERO_RANGE is faster in this case than writing
a bunch of zeros across a wire.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>