btrfs: stripe_meta: maintain trapped free space for metadata groups like data

btrfs: stripe_meta: maintain trapped free space for metadata groups like data

btrfs_block_group_init_stripe_unusable() arms stripe_meta groups and adds
their trapped bytes to the METADATA space_info, but the disarm, rescan
and commit-time scan only accept btrfs_is_stripe_alloc_bg() (DATA), so
a metadata group's contribution was frozen at its first value: never
updated as tree blocks were freed inside stripes, never dropped when the
group went read-only or was removed.  Tree blocks can only be placed in
fully free stripes, so the trapped bytes are as unusable as for data and
need the same maintenance.

Accept stripe_meta groups on the three maintenance paths, let the commit
scan sum METADATA space_infos, and apply the read-only subtraction to
the metadata branch of inc_block_group_ro() as well.

Assisted-by: Claude:claude-fable-5