switch xattr_handler->set() to passing dentry and inode separately

preparation for similar switch in ->setxattr() (see the next commit for
rationale).

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/posix_acl.c b/fs/posix_acl.c
index 2c60f17..8a4a266 100644
--- a/fs/posix_acl.c
+++ b/fs/posix_acl.c
@@ -822,10 +822,10 @@
 
 static int
 posix_acl_xattr_set(const struct xattr_handler *handler,
-		    struct dentry *dentry, const char *name,
-		    const void *value, size_t size, int flags)
+		    struct dentry *unused, struct inode *inode,
+		    const char *name, const void *value,
+		    size_t size, int flags)
 {
-	struct inode *inode = d_backing_inode(dentry);
 	struct posix_acl *acl = NULL;
 	int ret;