RISC-V: kexec: Fix memory leak of elf header buffer

In the Linux kernel, the following vulnerability has been resolved:

RISC-V: kexec: Fix memory leak of elf header buffer

This is reported by kmemleak detector:

unreferenced object 0xff2000000403d000 (size 4096):

comm "kexec", pid 146, jiffies 4294900633 (age 64.792s)

hex dump (first 32 bytes):

7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00 .ELF............

04 00 f3 00 01 00 00 00 00 00 00 00 00 00 00 00 ................

backtrace:

[<00000000566ca97c>] kmemleak_vmalloc+0x3c/0xbe

[<00000000979283d8>] __vmalloc_node_range+0x3ac/0x560

[<00000000b4b3712a>] __vmalloc_node+0x56/0x62

[<00000000854f75e2>] vzalloc+0x2c/0x34

[<00000000e9a00db9>] crash_prepare_elf64_headers+0x80/0x30c

[<0000000067e8bf48>] elf_kexec_load+0x3e8/0x4ec

[<0000000036548e09>] kexec_image_load_default+0x40/0x4c

[<0000000079fbe1b4>] sys_kexec_file_load+0x1c4/0x322

[<0000000040c62c03>] ret_from_syscall+0x0/0x2

In elf_kexec_load(), a buffer is allocated via vzalloc() to store elf

headers. While it's not freed back to system when kdump kernel is

reloaded or unloaded, or when image->elf_header is successfully set and

then fails to load kdump kernel for some reason. Fix it by freeing the

buffer in arch_kimage_file_post_load_cleanup().