s390x/ipl: check kernel command line size

s390x/ipl: check kernel command line size

Check if the provided kernel command line exceeds the maximum size of the s390x
Linux kernel command line size, which is 896 bytes.

Reported-by: Sven Schnelle <[email protected]>
Signed-off-by: Marc Hartmayer <[email protected]>
Message-Id: <20211006092631[email protected]>
Reviewed-by: Christian Borntraeger <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
[thuth: Adjusted format specifier for size_t]
Signed-off-by: Thomas Huth <[email protected]>