scoutfs: add kernelcompat files

Add files that we'll use to detect and work around incompatibilities
between kernel versions.

Signed-off-by: Zach Brown <zab@versity.com>

kmod/src/Makefile

@@ -4,6 +4,9 @@ CFLAGS_super.o = -DSCOUTFS_GIT_DESCRIBE=\"$(SCOUTFS_GIT_DESCRIBE)\" \
 		 -DSCOUTFS_FORMAT_HASH=0x$(SCOUTFS_FORMAT_HASH)LLU
 
 CFLAGS_scoutfs_trace.o = -I$(src) # define_trace.h double include
+
+# add EXTRA_CFLAGS defines for kernel compat
+-include $(src)/Makefile.kernelcompat
  
 scoutfs-y +=			\
 	bio.o			\

kmod/src/Makefile.kernelcompat

@@ -0,0 +1,7 @@
+#
+# We try to detect the specific api incompatibilities with simple tests
+# because distros regularly backport features without changing the
+# version.
+#
+
+ccflags-y += -include $(src)/kernelcompat.h

kmod/src/kernelcompat.h

@@ -0,0 +1,4 @@
+#ifndef _SCOUTFS_KERNELCOMPAT_H_
+#define _SCOUTFS_KERNELCOMPAT_H_
+
+#endif