arm64: elf: fix core dumping to match what glibc expects

The kernel's internal definition of ELF_NGREG uses struct pt_regs, which
means that we disagree with userspace on the size of coredumps since
glibc correctly uses the user-visible struct user_pt_regs.

This patch fixes our ELF_NGREG definition to use struct user_pt_regs
and introduces our own ELF_CORE_COPY_REGS to convert between the user
and kernel structure definitions.

Cc: <stable@vger.kernel.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/asm/elf.h

@@ -26,7 +26,10 @@
 
 typedef unsigned long elf_greg_t;
 
-#define ELF_NGREG (sizeof (struct pt_regs) / sizeof(elf_greg_t))
+#define ELF_NGREG (sizeof(struct user_pt_regs) / sizeof(elf_greg_t))
+#define ELF_CORE_COPY_REGS(dest, regs)	\
+	*(struct user_pt_regs *)&(dest) = (regs)->user_regs;
+
 typedef elf_greg_t elf_gregset_t[ELF_NGREG];
 typedef struct user_fpsimd_state elf_fpregset_t;