ACPI: bgrt: Fix CFI violation

clang's Control Flow Integrity requires that every indirect call has a
valid target, which is based on the type of the function pointer. The
*_show() functions in this file are written as if they will be called
from dev_attr_show(); however, they will be called from
sysfs_kf_seq_show() because the files were created by
sysfs_create_group() and the sysfs ops are based on kobj_sysfs_ops
because of kobject_add_and_create(). Because the *_show() functions do
not match the type of the show() member in struct kobj_attribute, there
is a CFI violation.

$ cat /sys/firmware/acpi/bgrt/{status,type,version,{x,y}offset}}
1
0
1
522
307

$ dmesg | grep "CFI failure"
[  267.761825] CFI failure (target: type_show.d5e1ad21498a5fd14edbc5c320906598.cfi_jt+0x0/0x8):
[  267.762246] CFI failure (target: xoffset_show.d5e1ad21498a5fd14edbc5c320906598.cfi_jt+0x0/0x8):
[  267.762584] CFI failure (target: status_show.d5e1ad21498a5fd14edbc5c320906598.cfi_jt+0x0/0x8):
[  267.762973] CFI failure (target: yoffset_show.d5e1ad21498a5fd14edbc5c320906598.cfi_jt+0x0/0x8):
[  267.763330] CFI failure (target: version_show.d5e1ad21498a5fd14edbc5c320906598.cfi_jt+0x0/0x8):

Convert these functions to the type of the show() member in struct
kobj_attribute so that there is no more CFI violation. Because these
functions are all so similar, combine them into a macro.

Fixes: d1ff4b1c

 ("ACPI: Add support for exposing BGRT data")
Link: https://github.com/ClangBuiltLinux/linux/issues/1406
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>