btrfs: stripe_alloc: keep the admitted bytes placeable when a group goes read-only

btrfs: stripe_alloc: keep the admitted bytes placeable when a group goes read-only

inc_block_group_ro() checks that the rest of the space_info can absorb
the group's free space in bytes.  Under stripe_alloc the data admission
gate promised writers whole stripes: bytes_may_use plus the held margin
against bytes_stripe_claimable.  Making a group read-only removes its
claimable stripes from that supply after the promise was made, so with
the reclaim worker relocating a group at the fill edge, admitted
writebacks found no stripe and were dropped:

  allocation failed flags 129, wanted 8192 ...
  space_info DATA has 450560 free, is full ... block group ... [readonly]

Refuse the read-only transition while the admitted bytes would no
longer fit in the claimable supply without this group.  Scrub, balance
and reclaim retry later; this is the fail-early direction and only bites
transiently at the edge, since bytes_may_use drains as writeback
completes.

Assisted-by: Claude:claude-fable-5