Fix: block_start removed from btrfs_get_extent in 6.11-rc1

Fix: block_start removed from btrfs_get_extent in 6.11-rc1

See upstream commit:

    commit c77a8c61002e91d859e118008fd495efbe1d9373
    Author: Qu Wenruo <wqu@suse.com>
    Date:   Tue Apr 30 07:53:06 2024 +0930

        btrfs: remove extent_map::block_start member

        The member extent_map::block_start can be calculated from
        extent_map::disk_bytenr + extent_map::offset for regular extents.
        And otherwise just extent_map::disk_bytenr.

        And this is already validated by the validate_extent_map().  Now we can
        remove the member.

        However there is a special case in btrfs_create_dio_extent() where we
        for NOCOW/PREALLOC ordered extents cannot directly use the resulting
        btrfs_file_extent, as btrfs_split_ordered_extent() cannot handle them
        yet.

        So for that call site, we pass file_extent->disk_bytenr +
        file_extent->num_bytes as disk_bytenr for the ordered extent, and 0 for
        offset.

Change-Id: I2e3245bb0d1f5263e902659aa05848d5e231909b
Signed-off-by: Kienan Stewart <kstewart@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>