fs: reduce work in fdget_pos()

1. predict the file was found
2. explicitly compare the ref to "one", ignoring the dead zone

The latter arguably improves the behavior to begin with. Suppose the
count turned bad -- the previously used ref routine is going to check
for it and return 0, indicating the count does not necessitate taking
->f_pos_lock. But there very well may be several users.

i.e. not paying for special-casing the dead zone improves semantics.

While here spell out each condition in a dedicated if statement. This
has no effect on generated code.

Sizes are as follows (in bytes; gcc 13, x86-64):
stock:		321
likely(): 	298
likely()+ref:	280

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://lore.kernel.org/r/20250319215801.1870660-1-mjguzik@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/file.c

@@ -1183,8 +1183,13 @@ struct fd fdget_raw(unsigned int fd)
  */
 static inline bool file_needs_f_pos_lock(struct file *file)
 {
-	return (file->f_mode & FMODE_ATOMIC_POS) &&
-		(file_count(file) > 1 || file->f_op->iterate_shared);
+	if (!(file->f_mode & FMODE_ATOMIC_POS))
+		return false;
+	if (__file_ref_read_raw(&file->f_ref) != FILE_REF_ONEREF)
+		return true;
+	if (file->f_op->iterate_shared)
+		return true;
+	return false;
 }
 
 struct fd fdget_pos(unsigned int fd)
@@ -1192,7 +1197,7 @@ struct fd fdget_pos(unsigned int fd)
 	struct fd f = fdget(fd);
 	struct file *file = fd_file(f);
 
-	if (file && file_needs_f_pos_lock(file)) {
+	if (likely(file) && file_needs_f_pos_lock(file)) {
 		f.word |= FDPUT_POS_UNLOCK;
 		mutex_lock(&file->f_pos_lock);
 	}

include/linux/file_ref.h

@@ -208,4 +208,18 @@ static inline unsigned long file_ref_read(file_ref_t *ref)
 	return c >= FILE_REF_RELEASED ? 0 : c + 1;
 }
 
+/*
+ * __file_ref_read_raw - Return the value stored in ref->refcnt
+ * @ref: Pointer to the reference count
+ *
+ * Return: The raw value found in the counter
+ *
+ * A hack for file_needs_f_pos_lock(), you probably want to use
+ * file_ref_read() instead.
+ */
+static inline unsigned long __file_ref_read_raw(file_ref_t *ref)
+{
+	return atomic_long_read(&ref->refcnt);
+}
+
 #endif