sep: Convert sep_lock_user_pages() to get_user_pages_fast()

CC: Mark Allyn <mark.a.allyn@intel.com>
CC: Jayant Mangalampalli <jayant.mangalampalli@intel.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>