btrfs: fix minimum number of chunk errors for DUP

The list of profiles in btrfs_chunk_max_errors lists DUP as a profile
DUP able to tolerate 1 device missing. Though this profile is special
with 2 copies, it still needs the device, unlike the others.

Looking at the history of changes, thre's no clear reason why DUP is
there, functions were refactored and blocks of code merged to one
helper.

d20983b4 Btrfs: fix writing data into the seed filesystem
  - factor code to a helper

de11cc12 Btrfs: don't pre-allocate btrfs bio
  - unrelated change, DUP still in the list with max errors 1

a236aed1

 Btrfs: Deal with failed writes in mirrored configurations
  - introduced the max errors, leaves DUP and RAID1 in the same group

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>