Btrfs: update stale comments referencing vmtruncate()

Commit a41ad394a03b ("Btrfs: convert to the new truncate sequence")
changed btrfs_setsize() to call truncate_setsize() instead of
vmtruncate() but didn't update the comment above it. truncate_setsize()
never fails (the IS_SWAPFILE() check happens elsewhere), so remove the
comment.

Additionally, the comment above btrfs_page_mkwrite() references
vmtruncate(), but truncate_setsize() does the size write and page
locking now.

Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -5369,7 +5369,6 @@ static int btrfs_setsize(struct inode *inode, struct iattr *attr)
 		if (ret)
 			return ret;
 
-		/* we don't support swapfiles, so vmtruncate shouldn't fail */
 		truncate_setsize(inode, newsize);
 
 		/* Disable nonlocked read DIO to avoid the end less truncate */
@@ -9044,8 +9043,8 @@ again:
  *
  * We are not allowed to take the i_mutex here so we have to play games to
  * protect against truncate races as the page could now be beyond EOF.  Because
- * vmtruncate() writes the inode size before removing pages, once we have the
- * page lock we can determine safely if the page is beyond EOF. If it is not
+ * truncate_setsize() writes the inode size before removing pages, once we have
+ * the page lock we can determine safely if the page is beyond EOF. If it is not
  * beyond EOF, then the page is guaranteed safe against truncation until we
  * unlock the page.
  */