btrfs: move split_extent_map to extent_map.c

split_extent_map doesn't have anything to do with the other code in
inode.c, so move it to extent_map.c.

This also allows marking replace_extent_mapping static.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>