staging: lustre: ptlrpc: remove unnecessary code

offset is an unsigned variable and, greater-than-or-equal-to-zero
comparison of an unsigned variable is always true.

Addresses-Coverity-ID: 1373919
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>