btrfs: increment extent count once when logging extents during fast fsync

btrfs: increment extent count once when logging extents during fast fsync

In btrfs_log_changed_extents() we increment the extent count twice, and
we then fallback to a transaction commit if the count reaches a threshold
of 32K. However we increment the count twice, which is confusing and
pointless. So increment the count only once and reduce the threshold to
half (16K).

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