fuse: fix argument type in fuse_get_user_pages()

Fix the following warning:

fs/fuse/file.c: In function 'fuse_direct_io':
fs/fuse/file.c:1002: warning: passing argument 3 of 'fuse_get_user_pages' from incompatible pointer type

This was introduced by commit f4975c67 "fuse: allow kernel to access
"direct_io" files".

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>