stable: clamp SUBLEVEL in 4.19

In a few months, SUBLEVEL will overflow, and some userspace may start
treating 4.19.256 as 4.20. While out of tree modules have different ways
of extracting the version number (and we're generally ok with breaking
them), we do care about breaking userspace and it would appear that this
overflow might do just that.

Our rules around userspace ABI in the stable kernel are pretty simple:
we don't break it. Thus, while userspace may be checking major/minor, it
shouldn't be doing anything with sublevel.

This patch applies a big band-aid to the 4.19 kernel in the form of
clamping the sublevel to 255.

The clamp is done for the purpose of LINUX_VERSION_CODE only, and
extracting the version number from the Makefile or "make kernelversion"
will continue to work as intended.

We might need to do it later in newer trees, but maybe we'll have a
better solution by then, so I'm ignoring that problem for now.

Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Makefile

@@ -1288,7 +1288,7 @@ endef
 
 define filechk_version.h
 	(echo \#define LINUX_VERSION_CODE $(shell                         \
-	expr $(VERSION) \* 65536 + 0$(PATCHLEVEL) \* 256 + 0$(SUBLEVEL)); \
+	expr $(VERSION) \* 65536 + 0$(PATCHLEVEL) \* 256 + 255); \
 	echo '#define KERNEL_VERSION(a,b,c) (((a) << 16) + ((b) << 8) + (c))';)
 endef