btrfs: update outdated comment at btrfs_replace_file_extents()

There is a comment at btrfs_replace_file_extents() that mentions that we
set the full sync flag on an inode when cloning into a file with a size
greater than or equals to 16MiB, through try_release_extent_mapping() when
we truncate the page cache after replacing file extents during a clone
operation.

That is not true anymore since commit 5e548b3201 ("btrfs: do not set
the full sync flag on the inode during page release"), so update the
comment to remove that part and rephrase it slightly to make it more
clear why the full sync flag is set at btrfs_replace_file_extents().

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/file.c

@@ -2765,10 +2765,8 @@ int btrfs_replace_file_extents(struct btrfs_inode *inode,
 	/*
 	 * If we were cloning, force the next fsync to be a full one since we
 	 * we replaced (or just dropped in the case of cloning holes when
-	 * NO_HOLES is enabled) extents and extent maps.
-	 * This is for the sake of simplicity, and cloning into files larger
-	 * than 16Mb would force the full fsync any way (when
-	 * try_release_extent_mapping() is invoked during page cache truncation.
+	 * NO_HOLES is enabled) file extent items and did not setup new extent
+	 * maps for the replacement extents (or holes).
 	 */
 	if (extent_info && !extent_info->is_new_extent)
 		set_bit(BTRFS_INODE_NEEDS_FULL_SYNC, &inode->runtime_flags);