btrfs: simplify the no-bioc fast path condition in btrfs_map_block

nr_alloc_stripes can't be one if we are writing to a replacement device,
as it is incremented for that case right above.  Remove the duplicate
checks.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>