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 <svens@linux.ibm.com>
Signed-off-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Message-Id: <20211006092631.20732-1-mhartmay@linux.ibm.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
[thuth: Adjusted format specifier for size_t]
Signed-off-by: Thomas Huth <thuth@redhat.com>