btrfs: move file_extent_item helpers into file-item.h

These helpers use functions that are in multiple places, which makes it
tricky to sync them into btrfs-progs.  Move them to file-item.h and then
include file-item.h in places that use these helpers.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>