commit 400db9d3 upstream.

remove 'len' variable in ext4_discard_allocated_blocks() because it is
useless.
Signed-off-by: Zheng Liu <wenqing.lz@taobao.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>