procfs: switch instantiate_t to d_splice_alias()

... and get rid of pointless struct inode *dir argument of those,
while we are at it.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/proc/internal.h b/fs/proc/internal.h
index 04a455b..275b062 100644
--- a/fs/proc/internal.h
+++ b/fs/proc/internal.h
@@ -154,7 +154,7 @@ extern struct dentry *proc_pid_lookup(struct inode *, struct dentry *, unsigned
 extern loff_t mem_lseek(struct file *, loff_t, int);
 
 /* Lookups */
-typedef int instantiate_t(struct inode *, struct dentry *,
+typedef struct dentry *instantiate_t(struct dentry *,
 				     struct task_struct *, const void *);
 extern bool proc_fill_cache(struct file *, struct dir_context *, const char *, int,
 			   instantiate_t, struct task_struct *, const void *);