Linux compat 2.6.37, invalidate_inodes()

In the 2.6.37 kernel the function invalidate_inodes() is no longer
exported for use by modules.  This memory management functionality
is needed to invalidate the inodes attached to a super block without
unmounting the filesystem.

Because this function still exists in the kernel and the prototype
is available is a common header all we strictly need is the symbol
address.  The address is obtained using spl_kallsyms_lookup_name()
and assigned to the variable invalidate_inodes_fn.  Then a #define
is used to replace all instances of invalidate_inodes() with a
call to the acquired address.  All the complexity is hidden behind
HAVE_INVALIDATE_INODES and invalidate_inodes() can be used as usual.

Long term we should try to get this, or another, interface made
available to modules again.

config/spl-build.m4

@@ -74,6 +74,7 @@ AC_DEFUN([SPL_AC_CONFIG_KERNEL], [
 	SPL_AC_KVASPRINTF
 	SPL_AC_3ARGS_FILE_FSYNC
 	SPL_AC_EXPORTED_RWSEM_IS_LOCKED
+	SPL_AC_KERNEL_INVALIDATE_INODES
 ])
 
 AC_DEFUN([SPL_AC_MODULE_SYMVERS], [
@@ -1685,3 +1686,18 @@ AC_DEFUN([SPL_AC_EXPORTED_RWSEM_IS_LOCKED], [
 		[rwsem_is_locked() acquires sem->wait_lock])],
 		[])
 ])
+
+dnl #
+dnl # 2.6.37 API compat,
+dnl # The function invalidate_inodes() is no longer exported by the kernel.
+dnl # The prototype however is still available which means it is safe
+dnl # to acquire the symbol's address using spl_kallsyms_lookup_name().
+dnl #
+AC_DEFUN([SPL_AC_KERNEL_INVALIDATE_INODES], [
+	SPL_CHECK_SYMBOL_EXPORT(
+		[invalidate_inodes],
+		[],
+		[AC_DEFINE(HAVE_INVALIDATE_INODES, 1,
+		[invalidate_inodes() is available])],
+		[])
+])