fs: turn some comments into kernel-doc

While reviewing ./include/linux/fs.h, I noticed that three comments can
actually be turned into kernel-doc comments. This allows to check the
consistency between the descriptions and the functions' signatures in
case they may change in the future.

A quick validation with the consistency check:

  ./scripts/kernel-doc -none include/linux/fs.h

currently reports no issues in this file.

Link: https://lore.kernel.org/r/20210204180059.28360-2-lukas.bulwahn@gmail.com
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

Author: bulwahn

include/linux/fs.h

@@ -1739,7 +1739,7 @@ static inline void sb_start_pagefault(struct super_block *sb)
 	__sb_start_write(sb, SB_FREEZE_PAGEFAULT);
 }
 
-/*
+/**
  * sb_start_intwrite - get write access to a superblock for internal fs purposes
  * @sb: the super we write to
  *
@@ -3161,7 +3161,7 @@ static inline ssize_t blockdev_direct_IO(struct kiocb *iocb,
 
 void inode_dio_wait(struct inode *inode);
 
-/*
+/**
  * inode_dio_begin - signal start of a direct I/O requests
  * @inode: inode the direct I/O happens on
  *
@@ -3173,7 +3173,7 @@ static inline void inode_dio_begin(struct inode *inode)
 	atomic_inc(&inode->i_dio_count);
 }
 
-/*
+/**
  * inode_dio_end - signal finish of a direct I/O requests
  * @inode: inode the direct I/O happens on
  *