fuse: convert fuse to use iov_iter_copy_[to|from]_user

A future patch hides the internals of struct iov_iter, so fuse should
be using the supported interface.

Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>
Acked-by: Miklos Szeredi <mszeredi@suse.cz>
Cc: fuse-devel@lists.sourceforge.net