btrfs: return whole extents in fiemap

  `xfs_io -c 'fiemap <off> <len>' <file>`

can give surprising results on btrfs that differ from xfs.

btrfs prints out extents trimmed to fit the user input. If the user's
fiemap request has an offset, then rather than returning each whole
extent which intersects that range, we also trim the start extent to not
have start < off.

Documentation in filesystems/fiemap.txt and the xfs_io man page suggests
that returning the whole extent is expected.

Some cases which all yield the same fiemap in xfs, but not btrfs:
  dd if=/dev/zero of=$f bs=4k count=1
  sudo xfs_io -c 'fiemap 0 1024' $f
    0: [0..7]: 26624..26631
  sudo xfs_io -c 'fiemap 2048 1024' $f
    0: [4..7]: 26628..26631
  sudo xfs_io -c 'fiemap 2048 4096' $f
    0: [4..7]: 26628..26631
  sudo xfs_io -c 'fiemap 3584 512' $f
    0: [7..7]: 26631..26631
  sudo xfs_io -c 'fiemap 4091 5' $f
    0: [7..6]: 26631..26630

I believe this is a consequence of the logic for merging contiguous
extents represented by separate extent items. That logic needs to track
the last offset as it loops through the extent items, which happens to
pick up the start offset on the first iteration, and trim off the
beginning of the full extent. To fix it, start `off` at 0 rather than
`start` so that we keep the iteration/merging intact without cutting off
the start of the extent.

after the fix, all the above commands give:

  0: [0..7]: 26624..26631

The merging logic is exercised by fstest generic/483, and I have written
a new fstest for checking we don't have backwards or zero-length fiemaps
for cases like those above.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Boris Burkov <boris@bur.io>
Signed-off-by: David Sterba <dsterba@suse.com>