rslib: Fix decoding of shortened codes

The decoding of shortenend codes is broken. It only works as expected if
there are no erasures.

When decoding with erasures, Lambda (the error and erasure locator
polynomial) is initialized from the given erasure positions. The pad
parameter is not accounted for by the initialisation code, and hence
Lambda is initialized from incorrect erasure positions.

The fix is to adjust the erasure positions by the supplied pad.

Signed-off-by: Ferdinand Blomqvist <ferdinand.blomqvist@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20190620141039.9874-3-ferdinand.blomqvist@gmail.com