arm64: If 'getrandom' syscall fails, don't error out - just warn and proceed.

For calculating the random 'kaslr-seed' value to be passed to the
secondary kernel (kexec or kdump), we invoke the 'getrandom' syscall
inside 'setup_2nd_dtb()' function.

Normally on most arm64 systems this syscall doesn't fail when the
initrd scriptware (which arms kdump service) invokes the same.
However, recently I noticed that on the 'hp-moonshot' arm64 boards,
we have an issue with the newer kernels which causes the same
to fail. As a result, the kdump service fails and we are not able
to use the kdump infrastructure just after boot. As expected, once the
random pool is sufficiently populated and we launch the kdump service
arming scripts again (manually), then the kdump service is properly
enabled.

Lets handle the same, by not error'ing out if 'getrandom' syscall fails.
Rather lets warn the user and proceed further by setting the
'kaslr-seed' value as 0 for the secondary kernel - which implies that it
boots in a 'nokaslr' mode.

Tested on my 'hp-moonshot' and 'qualcomm-amberwing' arm64 boards.

Signed-off-by: Bhupesh Sharma <bhsharma@redhat.com>
Signed-off-by: Simon Horman <horms@verge.net.au>