fs: Remove FIXME comment in generic_write_checks()

This patch removes an unnecessary comment that had to do with block special
files from `generic_write_checks()`.

The comment, originally added in Linux v2.4.14.9, was to clarify that we only
set `pos` to the file size when the file was opened with `O_APPEND` if the file
wasn't a block special file. Prior to Linux v2.4, block special files had a
different `write()` function which was unified into a generic `write()` function
in Linux v2.4. This generic `write()` function called `generic_write_checks()`.
For more details, see this earlier conversation:
https://lore.kernel.org/linux-fsdevel/Yc4Czk5A+p5p2Y4W@mit.edu/

Currently, block special devices have their own `write_iter()` function and no
longer share the same `generic_write_checks()`, therefore rendering the comment
irrelevant.
Signed-off-by: Tal Zussman <tz2294@columbia.edu>
Co-authored-by: Xijiao Li <xl2950@columbia.edu>
Co-authored-by: Hans Montero <hjm2133@columbia.edu>
Suggested-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/read_write.c

@@ -1637,7 +1637,6 @@ ssize_t generic_write_checks(struct kiocb *iocb, struct iov_iter *from)
 	if (!iov_iter_count(from))
 		return 0;
 
-	/* FIXME: this is for backwards compatibility with 2.4 */
 	if (iocb->ki_flags & IOCB_APPEND)
 		iocb->ki_pos = i_size_read(inode);