Btrfs: incremental send, fix file corruption when no-holes feature is enabled

When using the no-holes feature, if we have a file with prealloc extents
with a start offset beyond the file's eof, doing an incremental send can
cause corruption of the file due to incorrect hole detection. Such case
requires that the prealloc extent(s) exist in both the parent and send
snapshots, and that a hole is punched into the file that covers all its
extents that do not cross the eof boundary.

Example reproducer:

  $ mkfs.btrfs -f -O no-holes /dev/sdb
  $ mount /dev/sdb /mnt/sdb

  $ xfs_io -f -c "pwrite -S 0xab 0 500K" /mnt/sdb/foobar
  $ xfs_io -c "falloc -k 1200K 800K" /mnt/sdb/foobar

  $ btrfs subvolume snapshot -r /mnt/sdb /mnt/sdb/base

  $ btrfs send -f /tmp/base.snap /mnt/sdb/base

  $ xfs_io -c "fpunch 0 500K" /mnt/sdb/foobar

  $ btrfs subvolume snapshot -r /mnt/sdb /mnt/sdb/incr

  $ btrfs send -p /mnt/sdb/base -f /tmp/incr.snap /mnt/sdb/incr

  $ md5sum /mnt/sdb/incr/foobar
  816df6f64deba63b029ca19d880ee10a   /mnt/sdb/incr/foobar

  $ mkfs.btrfs -f /dev/sdc
  $ mount /dev/sdc /mnt/sdc

  $ btrfs receive -f /tmp/base.snap /mnt/sdc
  $ btrfs receive -f /tmp/incr.snap /mnt/sdc

  $ md5sum /mnt/sdc/incr/foobar
  cf2ef71f4a9e90c2f6013ba3b2257ed2   /mnt/sdc/incr/foobar

    --> Different checksum, because the prealloc extent beyond the
        file's eof confused the hole detection code and it assumed
        a hole starting at offset 0 and ending at the offset of the
        prealloc extent (1200Kb) instead of ending at the offset
        500Kb (the file's size).

Fix this by ensuring we never cross the file's size when issuing the
write operations for a hole.

Fixes: 16e7549f ("Btrfs: incompatible format change to remove hole extents")
CC: stable@vger.kernel.org # 3.14+
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>