Check for KALLSYMS

Check at ./configure time that the kernel was built with kallsyms
support.  If the kernel doesn't have CONFIG_KALLSYMS defined the
modules will still compile cleanly but will not be loadable.  So
we really want to catch this early during ./configure.  Note that
we do not require CONFIG_KALLSYMS_ALL but it may be safely defined.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes openzfs#6

config/spl-build.m4

@@ -82,6 +82,7 @@ AC_DEFUN([SPL_AC_CONFIG_KERNEL], [
 	SPL_AC_KERN_PATH_PARENT_HEADER
 	SPL_AC_KERN_PATH_PARENT_SYMBOL
 	SPL_AC_KERN_PATH_LOCKED
+	SPL_AC_CONFIG_KALLSYMS
 	SPL_AC_CONFIG_ZLIB_INFLATE
 	SPL_AC_CONFIG_ZLIB_DEFLATE
 	SPL_AC_2ARGS_ZLIB_DEFLATE_WORKSPACESIZE
@@ -2129,6 +2130,26 @@ AC_DEFUN([SPL_AC_KERN_PATH_LOCKED], [
 		[])
 ])
 
+dnl #
+dnl # /proc/kallsyms support,
+dnl # Verify the kernel has CONFIG_KALLSYMS support enabled.
+dnl #
+AC_DEFUN([SPL_AC_CONFIG_KALLSYMS], [
+	AC_MSG_CHECKING([whether CONFIG_KALLSYMS is defined])
+	SPL_LINUX_TRY_COMPILE([
+		#if !defined(CONFIG_KALLSYMS)
+		#error CONFIG_KALLSYMS not defined
+		#endif
+	],[ ],[
+		AC_MSG_RESULT([yes])
+	],[
+		AC_MSG_RESULT([no])
+		AC_MSG_ERROR([
+	*** This kernel does not include the required kallsyms support.
+	*** Rebuild the kernel with CONFIG_KALLSYMS=y set.])
+	])
+])
+
 dnl #
 dnl # zlib inflate compat,
 dnl # Verify the kernel has CONFIG_ZLIB_INFLATE support enabled.