Btrfs: don't insert useless holes when punching beyond the inode's size

Btrfs: don't insert useless holes when punching beyond the inode's size

If we punch beyond the size of an inode, we'll correctly remove any prealloc extents,
but we'll also insert file extent items representing holes (disk bytenr == 0) that start
with a key offset that lies beyond the inode's size and are not contiguous with the last
file extent item.

Example:

  $XFS_IO_PROG -f -c "truncate 118811" $SCRATCH_MNT/foo
  $XFS_IO_PROG -c "fpunch 582007 864596" $SCRATCH_MNT/foo
  $XFS_IO_PROG -c "pwrite -S 0x0d -b 39987 92267 39987" $SCRATCH_MNT/foo

btrfs-debug-tree output:

  item 4 key (257 INODE_ITEM 0) itemoff 15885 itemsize 160
	inode generation 6 transid 6 size 132254 block group 0 mode 100600 links 1
  item 5 key (257 INODE_REF 256) itemoff 15872 itemsize 13
	inode ref index 2 namelen 3 name: foo
  item 6 key (257 EXTENT_DATA 0) itemoff 15819 itemsize 53
	extent data disk byte 0 nr 0 gen 6
	extent data offset 0 nr 90112 ram 122880
	extent compression 0
  item 7 key (257 EXTENT_DATA 90112) itemoff 15766 itemsize 53
	extent data disk byte 12845056 nr 4096 gen 6
	extent data offset 0 nr 45056 ram 45056
	extent compression 2
  item 8 key (257 EXTENT_DATA 585728) itemoff 15713 itemsize 53
	extent data disk byte 0 nr 0 gen 6
	extent data offset 0 nr 860160 ram 860160
	extent compression 0

The last extent item, which represents a hole, is useless as it lies beyond the inode's
size.

Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
Signed-off-by: Josef Bacik <jbacik@fb.com>