fs.h: fix outdated comment about file flags

The __dentry_open function was removed in
commit <2a027e7a>("fold __dentry_open() into its sole caller").
Signed-off-by: Li Qiang <liq3ea@gmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

include/linux/fs.h

@@ -94,7 +94,7 @@ typedef int (dio_iodone_t)(struct kiocb *iocb, loff_t offset,
 
 /*
  * flags in file.f_mode.  Note that FMODE_READ and FMODE_WRITE must correspond
- * to O_WRONLY and O_RDWR via the strange trick in __dentry_open()
+ * to O_WRONLY and O_RDWR via the strange trick in do_dentry_open()
  */
 
 /* file is open for reading */