Btrfs: only write one super copy during fsync

During a tree-log commit for fsync, we've been writing at least
two copies of the super block and forcing them to disk.

The other filesystems write only one, and this change brings us on
par with them.  A full transaction commit will write all the super
copies, so we still have redundant info written on a regular
basis.

Signed-off-by: Chris Mason <chris.mason@oracle.com>