fs: Move many prototypes to pagemap.h

These functions are page cache functionality and don't need to be
declared in fs.h.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Miaohe Lin <linmiaohe@huawei.com>