btrfs: simplify the inline extent map creation

With the tree-checker ensuring all inline file extents starts at file
offset 0 and has a length no larger than sectorsize, we can simplify the
calculation to assigned those fixes values directly.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>