s390/kdump: Remove kzalloc_panic

For this function there are only two users, when 1) the elfcorehdr and 2)
the vmcoreinfo is allocated. However a missing vmcoreinfo is not critical
for kdump. So panicking when it cannot be allocated is not required.

Remove kzalloc_panic and adjust its callers accordingly.

Signed-off-by: Philipp Rudo <prudo@linux.ibm.com>
Acked-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>