btrfs: move ffe_ctl one level up

We are passing too many variables as it is from btrfs_reserve_extent() to
find_free_extent(). The next commit will add min_alloc_size to ffe_ctl, and
that means another pass-through argument. Take this opportunity to move
ffe_ctl one level up and drop the redundant arguments.

Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>