btrfs: defrag: don't try to merge regular extents with preallocated extents

[BUG]
With older kernels (before v5.16), btrfs will defrag preallocated extents.
While with newer kernels (v5.16 and newer) btrfs will not defrag
preallocated extents, but it will defrag the extent just before the
preallocated extent, even it's just a single sector.

This can be exposed by the following small script:

	mkfs.btrfs -f $dev > /dev/null

	mount $dev $mnt
	xfs_io -f -c "pwrite 0 4k" -c sync -c "falloc 4k 16K" $mnt/file
	xfs_io -c "fiemap -v" $mnt/file
	btrfs fi defrag $mnt/file
	sync
	xfs_io -c "fiemap -v" $mnt/file

The output looks like this on older kernels:

/mnt/btrfs/file:
 EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
   0: [0..7]:          26624..26631         8   0x0
   1: [8..39]:         26632..26663        32 0x801
/mnt/btrfs/file:
 EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
   0: [0..39]:         26664..26703        40   0x1

Which defrags the single sector along with the preallocated extent, and
replace them with an regular extent into a new location (caused by data
COW).
This wastes most of the data IO just for the preallocated range.

On the other hand, v5.16 is slightly better:

/mnt/btrfs/file:
 EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
   0: [0..7]:          26624..26631         8   0x0
   1: [8..39]:         26632..26663        32 0x801
/mnt/btrfs/file:
 EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
   0: [0..7]:          26664..26671         8   0x0
   1: [8..39]:         26632..26663        32 0x801

The preallocated range is not defragged, but the sector before it still
gets defragged, which has no need for it.

[CAUSE]
One of the function reused by the old and new behavior is
defrag_check_next_extent(), it will determine if we should defrag
current extent by checking the next one.

It only checks if the next extent is a hole or inlined, but it doesn't
check if it's preallocated.

On the other hand, out of the function, both old and new kernel will
reject preallocated extents.

Such inconsistent behavior causes above behavior.

[FIX]
- Also check if next extent is preallocated
  If so, don't defrag current extent.

- Add comments for each branch why we reject the extent

This will reduce the IO caused by defrag ioctl and autodefrag.

CC: stable@vger.kernel.org # 5.16
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>