Btrfs: fix wrong check for btrfs_force_chunk_alloc()

btrfs_force_chunk_alloc() return 1 for allocation chunk successfully.
This problem exists since commit c87f08ca.

With this patch, we might fix some enospc problems for balances.

Signed-off-by: Wang Shilong <wangshilong1991@gmail.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Tested-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>