vfs: make it possible to access the dentry hash/len as one 64-bit entry

This allows comparing hash and len in one operation on 64-bit
architectures.  Right now only __d_lookup_rcu() takes advantage of this,
since that is the case we care most about.

The use of anonymous struct/unions hides the alternate 64-bit approach
from most users, the exception being a few cases where we initialize a
'struct qstr' with a static initializer.  This makes the problematic
cases use a new QSTR_INIT() helper function for that (but initializing
just the name pointer with a "{ .name = xyzzy }" initializer remains
valid, as does just copying another qstr structure).

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/ext4/namei.c

@@ -1052,10 +1052,7 @@ static struct dentry *ext4_lookup(struct inode *dir, struct dentry *dentry, stru
 struct dentry *ext4_get_parent(struct dentry *child)
 {
 	__u32 ino;
-	static const struct qstr dotdot = {
-		.name = "..",
-		.len = 2,
-	};
+	static const struct qstr dotdot = QSTR_INIT("..", 2);
 	struct ext4_dir_entry_2 * de;
 	struct buffer_head *bh;