The variable ret is being assigned a value that is never read, the
error exit path via label 'unmap' returns -ENOMEM anyhow, so assigning
ret with -ENOMEM is redundamt.

Addresses-Coverity: ("Unused value")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>