s390/kexec: Emit an error message when cmdline is too long

s390/kexec: Emit an error message when cmdline is too long

Currently, if the command line passed to kexec_file_load() exceeds
the supported limit of the kernel being kexec'd, -EINVAL is returned
to userspace, which is consistent across architectures. Since
-EINVAL is not specific to this case, the kexec tool cannot provide
a specific reason for the failure. Many architectures emit an error
message in this case. Add a similar error message, including the
effective limit, since the command line length is configurable.

Acked-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>