btrfs: fix wrong block_start calculation for btrfs_drop_extent_map_range()

[BUG]
During my extent_map cleanup/refactor, with extra sanity checks,
extent-map-tests::test_case_7() would not pass the checks.

The problem is, after btrfs_drop_extent_map_range(), the resulted
extent_map has a @block_start way too large.
Meanwhile my btrfs_file_extent_item based members are returning a
correct @disk_bytenr/@offset combination.

The extent map layout looks like this:

     0        16K    32K       48K
     | PINNED |      | Regular |

The regular em at [32K, 48K) also has 32K @block_start.

Then drop range [0, 36K), which should shrink the regular one to be
[36K, 48K).
However the @block_start is incorrect, we expect 32K + 4K, but got 52K.

[CAUSE]
Inside btrfs_drop_extent_map_range() function, if we hit an extent_map
that covers the target range but is still beyond it, we need to split
that extent map into half:

	|<-- drop range -->|
		 |<----- existing extent_map --->|

And if the extent map is not compressed, we need to forward
extent_map::block_start by the difference between the end of drop range
and the extent map start.

However in that particular case, the difference is calculated using
(start + len - em->start).

The problem is @start can be modified if the drop range covers any
pinned extent.

This leads to wrong calculation, and would be caught by my later
extent_map sanity checks, which checks the em::block_start against
btrfs_file_extent_item::disk_bytenr + btrfs_file_extent_item::offset.

This is a regression caused by commit c962098c ("btrfs: fix
incorrect splitting in btrfs_drop_extent_map_range"), which removed the
@len update for pinned extents.

[FIX]
Fix it by avoiding using @start completely, and use @end - em->start
instead, which @end is exclusive bytenr number.

And update the test case to verify the @block_start to prevent such
problem from happening.

Thankfully this is not going to lead to any data corruption, as IO path
does not utilize btrfs_drop_extent_map_range() with @skip_pinned set.

So this fix is only here for the sake of consistency/correctness.

CC: stable@vger.kernel.org # 6.5+
Fixes: c962098c ("btrfs: fix incorrect splitting in btrfs_drop_extent_map_range")
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_map.c

@@ -817,7 +817,7 @@ void btrfs_drop_extent_map_range(struct btrfs_inode *inode, u64 start, u64 end,
 					split->block_len = em->block_len;
 					split->orig_start = em->orig_start;
 				} else {
-					const u64 diff = start + len - em->start;
+					const u64 diff = end - em->start;
 
 					split->block_len = split->len;
 					split->block_start += diff;

fs/btrfs/tests/extent-map-tests.c

@@ -847,6 +847,11 @@ static int test_case_7(struct btrfs_fs_info *fs_info)
 		goto out;
 	}
 
+	if (em->block_start != SZ_32K + SZ_4K) {
+		test_err("em->block_start is %llu, expected 36K", em->block_start);
+		goto out;
+	}
+
 	free_extent_map(em);
 
 	read_lock(&em_tree->lock);