f2fs: Get the superblock from the mapping instead of the page

It's slightly more efficient to go directly from the mapping to the
superblock than to go from the page.  Now that these routines have
the mapping passed to them, there's no reason not to use it.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Al Viro <viro@zeniv.linux.org.uk>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>