btrfs-progs: extent-tree: Enhance btrfs_record_file_extent

btrfs-progs: extent-tree: Enhance btrfs_record_file_extent

Btrfs_record_file_extent() has some small problems like:
1) Can't handle overlapping extents
2) May create extent larger than BTRFS_MAX_EXTENT_SIZE

So enhance it using previously added facilites.
This is used for later btrfs-convert, as for new convert, we create
saved image first, then copy inode.
Which will also cause extent overlapping.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>