Btrfs: catch errors from btrfs_sync_log

btrfs_sync_log returns -EAGAIN when we need full transaction commits
instead of small log commits, but sometimes we were dropping the return
value.

In practice, we check for this a few different ways, but this is still a
bug that can leave off full log commits when we really need them.

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