s390/crash: use the correct type for memory allocation

get_elfcorehdr_size() returns a size_t, so there is no real point to
store it in a u32.

Turn 'alloc_size' into a size_t.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/0756118c9058338f3040edb91971d0bfd100027b.1686688212.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>