proc: faster /proc/$PID lookup

Currently lookup for /proc/$PID first goes through spinlock and whole list
of misc /proc entries only to confirm that, yes, /proc/42 can not possibly
match random proc entry.

List is is several dozens entries long (52 entries on my setup).

None of this is necessary.

Try to convert dentry name to integer first.
If it works, it must be /proc/$PID.
If it doesn't, it must be random proc entry.

Based on patch from Al Viro.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/base.c

@@ -2780,7 +2780,7 @@ out:
 
 struct dentry *proc_pid_lookup(struct inode *dir, struct dentry * dentry, unsigned int flags)
 {
-	int result = 0;
+	int result = -ENOENT;
 	struct task_struct *task;
 	unsigned tgid;
 	struct pid_namespace *ns;

fs/proc/root.c

@@ -199,10 +199,10 @@ static int proc_root_getattr(struct vfsmount *mnt, struct dentry *dentry, struct
 
 static struct dentry *proc_root_lookup(struct inode * dir, struct dentry * dentry, unsigned int flags)
 {
-	if (!proc_lookup(dir, dentry, flags))
+	if (!proc_pid_lookup(dir, dentry, flags))
 		return NULL;
 	
-	return proc_pid_lookup(dir, dentry, flags);
+	return proc_lookup(dir, dentry, flags);
 }
 
 static int proc_root_readdir(struct file *file, struct dir_context *ctx)