fs/dcache: move security_d_instantiate() behind attaching dentry to inode

BugLink: https://bugs.launchpad.net/bugs/1852110

During backport 1e2e547a "do d_instantiate/unlock_new_inode
combinations safely", there was a error instantiating sequence of
attaching dentry to inode and calling security_d_instantiate().

Before commit ce23e640 "->getxattr(): pass dentry and inode as
separate arguments" and b9680917 "security_d_instantiate(): move to
the point prior to attaching dentry to inode", security_d_instantiate()
should be called beind __d_instantiate(), otherwise it will trigger
below problem when CONFIG_SECURITY_SMACK on ext4 was enabled because
d_inode(dentry) used by ->getxattr() is NULL before __d_instantiate()
instantiate inode.

[   31.858026] BUG: unable to handle kernel paging request at ffffffffffffff70
...
[   31.882024] Call Trace:
[   31.882378]  [<ffffffffa347f75c>] ext4_xattr_get+0x8c/0x3e0
[   31.883195]  [<ffffffffa3489454>] ext4_xattr_security_get+0x24/0x40
[   31.884086]  [<ffffffffa336a56b>] generic_getxattr+0x5b/0x90
[   31.884907]  [<ffffffffa3700514>] smk_fetch+0xb4/0x150
[   31.885634]  [<ffffffffa3700772>] smack_d_instantiate+0x1c2/0x550
[   31.886508]  [<ffffffffa36f9a5a>] security_d_instantiate+0x3a/0x80
[   31.887389]  [<ffffffffa3353b26>] d_instantiate_new+0x36/0x130
[   31.888223]  [<ffffffffa342b1ef>] ext4_mkdir+0x4af/0x6a0
[   31.888928]  [<ffffffffa3343470>] vfs_mkdir+0x100/0x280
[   31.889536]  [<ffffffffa334b086>] SyS_mkdir+0xb6/0x170
[   31.890255]  [<ffffffffa307c855>] ? trace_do_page_fault+0x95/0x2b0
[   31.891134]  [<ffffffffa3c5e078>] entry_SYSCALL_64_fastpath+0x18/0x73

Cc: <stable@vger.kernel.org> # 3.16, 4.4
Signed-off-by: zhangyi (F) <yi.zhang@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

fs/dcache.c

@@ -1904,7 +1904,6 @@ void d_instantiate_new(struct dentry *entry, struct inode *inode)
 	BUG_ON(!hlist_unhashed(&entry->d_u.d_alias));
 	BUG_ON(!inode);
 	lockdep_annotate_inode_mutex_key(inode);
-	security_d_instantiate(entry, inode);
 	spin_lock(&inode->i_lock);
 	__d_instantiate(entry, inode);
 	WARN_ON(!(inode->i_state & I_NEW));
@@ -1912,6 +1911,7 @@ void d_instantiate_new(struct dentry *entry, struct inode *inode)
 	smp_mb();
 	wake_up_bit(&inode->i_state, __I_NEW);
 	spin_unlock(&inode->i_lock);
+	security_d_instantiate(entry, inode);
 }
 EXPORT_SYMBOL(d_instantiate_new);