fuse: Convert fuse_write_begin() to use a folio

fuse: Convert fuse_write_begin() to use a folio

Fetch a folio from the page cache instead of a page and use it throughout
removing several calls to compound_head() and supporting large folios
(in this function).  We still have to convert back to a page for calling
internal fuse functions, but hopefully they will be converted soon.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>