fuse: writepages: update bdi writeout when deleting secondary request

BDI_WRITTEN counter is used to estimate bdi bandwidth.  It must be
incremented every time as bdi ends page writeback.  No matter whether it
was fulfilled by actual write or by discarding the request (e.g. due to
shrunk i_size).

Note that even before writepages patches, the case "Got truncated off
completely" was handled in fuse_send_writepage() by calling
fuse_writepage_finish() which updated BDI_WRITTEN unconditionally.

Signed-off-by: Maxim Patlasov <MPatlasov@parallels.com>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>