The write zeros command does not work with 4k range.

bash-4.4# ./blkdiscard /dev/nvme0n1p2
bash-4.4# strace -efallocate xfs_io -c "fzero 536895488 2048" /dev/nvme0n1p2
fallocate(3, FALLOC_FL_ZERO_RANGE, 536895488, 2048) = 0
+++ exited with 0 +++
bash-4.4# dd bs=1 if=/dev/nvme0n1p2 skip=536895488 count=512 | hexdump -C
00000000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00000200

bash-4.4# ./blkdiscard /dev/nvme0n1p2
bash-4.4# strace -efallocate xfs_io -c "fzero 536895488 4096" /dev/nvme0n1p2
fallocate(3, FALLOC_FL_ZERO_RANGE, 536895488, 4096) = 0
+++ exited with 0 +++
bash-4.4# dd bs=1 if=/dev/nvme0n1p2 skip=536895488 count=512 | hexdump -C
00000000  5c 61 5c b0 96 21 1b 5e  85 0c 07 32 9c 8c eb 3c  |\a\..!.^...2...<|
00000010  4a a2 06 ca 67 15 2d 8e  29 8d a8 a0 7e 46 8c 62  |J...g.-.)...~F.b|
00000020  bb 4c 6c c1 6b f5 ae a5  e4 a9 bc 93 4f 60 ff 7a  |.Ll.k.......O`.z|
Reported-by: Eric Sandeen <esandeen@redhat.com>
Signed-off-by: David Milburn <dmilburn@redhat.com>
Tested-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>