btrfs-progs: Fix wrong tree block alignment for unalianged block group

btrfs-progs: Fix wrong tree block alignment for unalianged block group

Commit 854437ca(btrfs-progs: extent-tree: avoid allocating tree block
that crosses stripe boundary) introduces check for logical bytenr not
crossing stripe boundary.

However that check is not completely correct.
It only checks if the logical bytenr and length agaist absolute logical
offset.
That's to say, it only check if a tree block lies in 64K logical stripe.

But in fact, it's possible a block group starts at bytenr unaligned with
64K, just like the following case.

Then btrfsck will give false alert.

0       32K       64K       96K        128K         160K ...
        |--------------- Block group A ---------------------
	|<-----TB 32K------>|
        |/Scrub stripe unit/|
|    WRONG UNIT   |

In that case, TB(tree block) at bytenr 32K in fact fits into the kernel
scrub stripe unit.
But doesn't fit into the pure logical 64K stripe.

Fix check_crossing_stripes() to compare bytenr to block group start, not
to absolute logical bytenr.

Reported-by: Jussi Kansanen <jussi.kansanen@gmail.com>
Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>