ext4: make FIEMAP and delayed allocation play well together

Fix the FIEMAP ioctl so that it returns all of the page ranges which
are still subject to delayed allocation.  We were missing some cases
if the file was sparse.

Reported by Chris Mason <chris.mason@oracle.com>:
>We've had reports on btrfs that cp is giving us files full of zeros
>instead of actually copying them.  It was tracked down to a bug with
>the btrfs fiemap implementation where it was returning holes for
>delalloc ranges.
>
>Newer versions of cp are trusting fiemap to tell it where the holes
>are, which does seem like a pretty neat trick.
>
>I decided to give xfs and ext4 a shot with a few tests cases too, xfs
>passed with all the ones btrfs was getting wrong, and ext4 got the basic
>delalloc case right.
>$ mkfs.ext4 /dev/xxx
>$ mount /dev/xxx /mnt
>$ dd if=/dev/zero of=/mnt/foo bs=1M count=1
>$ fiemap-test foo
>ext:   0 logical: [       0..     255] phys:        0..     255
>flags: 0x007 tot: 256
>
>Horray!  But once we throw a hole in, things go bad:
>$ mkfs.ext4 /dev/xxx
>$ mount /dev/xxx /mnt
>$ dd if=/dev/zero of=/mnt/foo bs=1M count=1 seek=1
>$ fiemap-test foo
>< no output >
>
>We've got a delalloc extent after the hole and ext4 fiemap didn't find
>it.  If I run sync to kick the delalloc out:
>$sync
>$ fiemap-test foo
>ext:   0 logical: [     256..     511] phys:    34048..   34303
>flags: 0x001 tot: 256
>
>fiemap-test is sitting in my /usr/local/bin, and I have no idea how it
>got there.  It's full of pretty comments so I know it isn't mine, but
>you can grab it here:
>
>http://oss.oracle.com/~mason/fiemap-test.c


>
>xfsqa has a fiemap program too.

After Fix, test results are as follows:
ext:   0 logical: [     256..     511] phys:        0..     255
flags: 0x007 tot: 256
ext:   0 logical: [     256..     511] phys:    33280..   33535
flags: 0x001 tot: 256

$ mkfs.ext4 /dev/xxx
$ mount /dev/xxx /mnt
$ dd if=/dev/zero of=/mnt/foo bs=1M count=1 seek=1
$ sync
$ dd if=/dev/zero of=/mnt/foo bs=1M count=1 seek=3
$ dd if=/dev/zero of=/mnt/foo bs=1M count=1 seek=5
$ fiemap-test foo
ext:   0 logical: [     256..     511] phys:    33280..   33535
flags: 0x000 tot: 256
ext:   1 logical: [     768..    1023] phys:        0..     255
flags: 0x006 tot: 256
ext:   2 logical: [    1280..    1535] phys:        0..     255
flags: 0x007 tot: 256

Tested-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Signed-off-by: Yongqiang Yang <xiaoqiangnk@gmail.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>