Linux 3.4 compat, d_make_root() replaces d_alloc_root()

torvalds/linux@adc0e91ab1 introduced
introduced d_make_root() as a replacement for d_alloc_root(). Further
commits appear to have removed d_alloc_root() from the Linux source
tree. This causes the following failure:

  error: implicit declaration of function 'd_alloc_root'
  [-Werror=implicit-function-declaration]

To correct this we update the code to use the current d_make_root()
interface for readability.  Then we introduce an autotools check
to determine if d_make_root() is available.  If it isn't then we
define some compatibility logic which used the older d_alloc_root()
interface.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #776

config/kernel-d-make-root.m4

@@ -0,0 +1,12 @@
+dnl #
+dnl # 3.4.0 API change
+dnl # Added d_make_root() to replace previous d_alloc_root() function.
+dnl #
+AC_DEFUN([ZFS_AC_KERNEL_D_MAKE_ROOT], [
+	ZFS_CHECK_SYMBOL_EXPORT(
+		[d_make_root],
+		[fs/dcache.c],
+		[AC_DEFINE(HAVE_D_MAKE_ROOT, 1,
+		[d_make_root() is available])],
+		[])
+])

config/kernel.m4

@@ -48,6 +48,7 @@ AC_DEFUN([ZFS_AC_CONFIG_KERNEL], [
 	ZFS_AC_KERNEL_CREATE_UMODE_T
 	ZFS_AC_KERNEL_AUTOMOUNT
 	ZFS_AC_KERNEL_INSERT_INODE_LOCKED
+	ZFS_AC_KERNEL_D_MAKE_ROOT
 	ZFS_AC_KERNEL_D_OBTAIN_ALIAS
 	ZFS_AC_KERNEL_CHECK_DISK_SIZE_CHANGE
 	ZFS_AC_KERNEL_TRUNCATE_SETSIZE