Btrfs: fix type mismatch in find_free_extent()

data parameter should be u64 because a full-sized chunk flags field is
passed instead of 0/1 for distinguishing data from metadata.  All
underlying functions expect u64.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>