btrfs: Remove redundant checks from btrfs_alloc_data_chunk_ondemand

Many commits ago the data space_info in alloc_data_chunk_ondemand used to be
acquired from the inode. At that point commit
33b4d47f ("Btrfs: deal with NULL space info") got introduced to deal with
spurios cases where the space info could be null, following a rebalance.
Nowadays, however, the space info is referenced directly from the btrfs_fs_info
struct which is initialised at filesystem mount time. This makes the null
checks redundant, so remove them.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>