scoutfs: require sparse builds

Now that we know that it's easy to fix sparse build failures against
RHEL kernel headers we can require sparse builds when developing.

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

kmod/Makefile

@@ -1,4 +1,33 @@
 ALL: module
 
+#
+# SK_KSRC points to the kernel header build dir to build against.
+# On a running machine this could be /lib/modules/$(uname -r)/build with
+# the right kernel-headers package installed.  I tend to build on other
+# hosts so I extract the kernel-headers package for the target machine's
+# kernel in a dir somehere.
+#
+# sparse is critical for avoiding endian mistakes.  It should just work
+# if the sparse package is installed.
+#
+# but sometimes kernel-headers are broken.    For example, the
+# rhel 3.10.0-327.el7.x86_64 kernel needs the following patch.
+# We'll try to have a git tree with fixed headers.
+#
+#
+# diff --git a/include/linux/rh_kabi.h b/include/linux/rh_kabi.h
+# index 1767770..0a8e5f3 100644
+# --- a/include/linux/rh_kabi.h
+# +++ b/include/linux/rh_kabi.h
+# @@ -73,7 +73,6 @@
+#                 struct {                                \
+#                         _orig;                          \
+#                 } __UNIQUE_ID(rh_kabi_hide);            \
+# -               __RH_KABI_CHECK_SIZE_ALIGN(_orig, _new);        \
+#         }
+#
+#  #define _RH_KABI_REPLACE_UNSAFE(_orig, _new)   _new
+
 module:
 	make CONFIG_SCOUTFS_FS=m -C $(SK_KSRC) M=$(PWD)/src
+	make C=2 CF="-D__CHECK_ENDIAN__" CONFIG_SCOUTFS_FS=m -C $(SK_KSRC) M=$(PWD)/src