generic: fix compiler error in OpenWrt's kexec patch

generic: fix compiler error in OpenWrt's kexec patch

330-MIPS-kexec-Accept-command-line-parameters-from-users.patch causes
problems when building with -Werror=unused-result.

arch/mips/kernel/machine_kexec.c: In function 'machine_kexec_init_argv':
arch/mips/kernel/machine_kexec.c:76:2: error: ignoring return value of 'copy_from_user', declared with attribute warn_unused_result [-Werror=unused-result]
  copy_from_user(kexec_argv_buf, buf, size);
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors

Fix this by handling the return value in an appropriate way.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>