cleanup sync_supers

Merge the write_super helper into sync_super and move the check for
->write_super earlier so that we can avoid grabbing a reference to
a superblock that doesn't have it.

While we're at it also add a little comment documenting sync_supers.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>