btrfs: don't allow log trees to consume global reserve or overcommit metadata

For a fsync we never reserve space in advance, we just start a transaction
without reserving space and we use an empty block reserve for a log tree.
We reserve space as we need while updating a log tree, we end up in
btrfs_use_block_rsv() when reserving space for the allocation of a log
tree extent buffer and we attempt first to reserve without flushing,
and if that fails we attempt to consume from the global reserve or
overcommit metadata. This makes us consume space that may be the last
resort for a transaction commit to succeed, therefore increasing the
chances for a transaction abort with -ENOSPC.

So make btrfs_use_block_rsv() fail if we can't reserve metadata space for
a log tree extent buffer allocation without flushing, making the fsync
fallback to a transaction commit and avoid using critical space that could
be the only resort for a transaction commit to succeed when we are in a
critical space situation.

Reviewed-by: Leo Martins <loemra.dev@gmail.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

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