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.

Change-Id: I18158fd4fe9011915698a492565762f3a399e1c6
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>
Acked-by: Miklos Szeredi <mszeredi@suse.cz>
Cc: fuse-devel@lists.sourceforge.net