arm64: kexec_file: Use struct_size() in kmalloc()

Make use of the struct_size() helper instead of an open-coded version
in order to avoid any potential type mistakes.

This code was detected with the help of Coccinelle and, audited and
fixed manually.
Signed-off-by: NGustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20200617213407.GA1385@embeddedorSigned-off-by: NWill Deacon <will@kernel.org>

arch/arm64/kernel/machine_kexec_file.c

@@ -219,8 +219,7 @@ static int prepare_elf_headers(void **addr, unsigned long *sz)
 					MEMBLOCK_NONE, &start, &end, NULL)
 		nr_ranges++;
 
-	cmem = kmalloc(sizeof(struct crash_mem) +
-			sizeof(struct crash_mem_range) * nr_ranges, GFP_KERNEL);
+	cmem = kmalloc(struct_size(cmem, ranges, nr_ranges), GFP_KERNEL);
 	if (!cmem)
 		return -ENOMEM;