powerpc/ftrace: Fix dropping weak symbols with older toolchains

The minimum level of gcc supported for building the kernel is v5.1.
v5.x releases of gcc emitted a three instruction sequence for
-mprofile-kernel:
	mflr	r0
	std	r0, 16(r1)
	bl	_mcount

It is only with the v6.x releases that gcc started emitting the two
instruction sequence for -mprofile-kernel, omitting the second store
instruction.

With the older three instruction sequence, the actual ftrace location
can be the 5th instruction into a function. Update the allowed offset
for ftrace location from 12 to 16 to accommodate the same.

Cc: stable@vger.kernel.org
Fixes: 7af82ff ("powerpc/ftrace: Ignore weak functions")
Signed-off-by: Naveen N Rao <naveen@kernel.org>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/7b265908a9461e38fc756ef9b569703860a80621.1687166935.git.naveen@kernel.org

Author: rnav

arch/powerpc/include/asm/ftrace.h

@@ -12,7 +12,7 @@
 
 /* Ignore unused weak functions which will have larger offsets */
 #ifdef CONFIG_MPROFILE_KERNEL
-#define FTRACE_MCOUNT_MAX_OFFSET	12
+#define FTRACE_MCOUNT_MAX_OFFSET	16
 #elif defined(CONFIG_PPC32)
 #define FTRACE_MCOUNT_MAX_OFFSET	8
 #endif