platform/x86: dell-smbios: fix string overflow

The new sysfs code overwrites two fixed-length character arrays
that are each one byte shorter than they need to be, to hold
the trailing \0:

drivers/platform/x86/dell-smbios.c: In function 'build_tokens_sysfs':
drivers/platform/x86/dell-smbios.c:494:42: error: 'sprintf' writing a terminating nul past the end of the destination [-Werror=format-overflow=]
   sprintf(buffer_location, "%04x_location",
drivers/platform/x86/dell-smbios.c:494:3: note: 'sprintf' output 14 bytes into a destination of size 13
drivers/platform/x86/dell-smbios.c:506:36: error: 'sprintf' writing a terminating nul past the end of the destination [-Werror=format-overflow=]
   sprintf(buffer_value, "%04x_value",
drivers/platform/x86/dell-smbios.c:506:3: note: 'sprintf' output 11 bytes into a destination of size 10

This changes it to just use kasprintf(), which always gets it right.

Discovered with gcc-7.1.1 with the following commit reverted:
bd664f6b disable new gcc-7.1.1 warnings for now

Fixes: 33b9ca1e ("platform/x86: dell-smbios: Add a sysfs interface for SMBIOS tokens")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Mario Limonciello <mario.limonciello@dell.com>
[dvhart: add subject prefix and reproducer details for context]
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>