change how d_alias is replaced by du.d_alias

d_alias may need to be converted to du.d_alias
depending on the kernel version.
d_alias is currently in only one place in the code which
changes
"hlist_for_each_entry(dentry, &inode->i_dentry, d_alias)"
to
"hlist_for_each_entry(dentry, &inode->i_dentry, d_u.d_alias)"
as neccesary.

This effectively results in a double macro expansion
for code that uses the zfs headers but already has its
own macro for just d_alias (lustre in this case).

Remove the conditional code for hlist_for_each_entry
and have a macro for "d_alias -> du.d_alias" instead.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Gian-Carlo DeFazio <defazio1@llnl.gov>
Closes #14377

Author: defaziogiancarlo

include/os/linux/kernel/linux/dcache_compat.h

@@ -35,6 +35,10 @@
 #define	d_make_root(inode)	d_alloc_root(inode)
 #endif /* HAVE_D_MAKE_ROOT */
 
+#ifdef HAVE_DENTRY_D_U_ALIASES
+#define	d_alias			d_u.d_alias
+#endif
+
 /*
  * 2.6.30 API change,
  * The const keyword was added to the 'struct dentry_operations' in
@@ -70,11 +74,7 @@ zpl_d_drop_aliases(struct inode *inode)
 {
 	struct dentry *dentry;
 	spin_lock(&inode->i_lock);
-#ifdef HAVE_DENTRY_D_U_ALIASES
-	hlist_for_each_entry(dentry, &inode->i_dentry, d_u.d_alias) {
-#else
 	hlist_for_each_entry(dentry, &inode->i_dentry, d_alias) {
-#endif
 		if (!IS_ROOT(dentry) && !d_mountpoint(dentry) &&
 		    (dentry->d_inode == inode)) {
 			d_drop(dentry);