btrfs: open code need_full_stripe conditions

need_full_stripe is just a somewhat complicated way to say
"op != BTRFS_MAP_READ".  Just spell that explicit check out, which makes
a lot of the code currently using the helper easier to understand.

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>