btrfs: fix the comment on lock_extent_buffer_for_io

The return value of that function is completely wrong.

That function only returns 0 if the extent buffer doesn't need to be
submitted.  The "ret = 1" and "ret = 0" are determined by the return
value of "test_and_clear_bit(EXTENT_BUFFER_DIRTY, &eb->bflags)".

And if we get ret == 1, it's because the extent buffer is dirty, and we
set its status to EXTENT_BUFFER_WRITE_BACK, and continue to page
locking.

While if we get ret == 0, it means the extent is not dirty from the
beginning, so we don't need to write it back.

The caller also follows this, in btree_write_cache_pages(), if
lock_extent_buffer_for_io() returns 0, we just skip the extent buffer
completely.

So the comment is completely wrong.

Since we're here, also change the description a little.  The write bio
flushing won't be visible to the caller, thus it's not an major feature.
In the main description, only describe the locking part to make the
point more clear.

For reference, added in commit 2e3c2513 ("btrfs: extent_io: add
proper error handling to lock_extent_buffer_for_io()")
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -3652,11 +3652,14 @@ static void end_extent_buffer_writeback(struct extent_buffer *eb)
 }
 
 /*
- * Lock eb pages and flush the bio if we can't the locks
+ * Lock extent buffer status and pages for writeback.
  *
- * Return  0 if nothing went wrong
- * Return >0 is same as 0, except bio is not submitted
- * Return <0 if something went wrong, no page is locked
+ * May try to flush write bio if we can't get the lock.
+ *
+ * Return  0 if the extent buffer doesn't need to be submitted.
+ *           (E.g. the extent buffer is not dirty)
+ * Return >0 is the extent buffer is submitted to bio.
+ * Return <0 if something went wrong, no page is locked.
  */
 static noinline_for_stack int lock_extent_buffer_for_io(struct extent_buffer *eb,
 			  struct extent_page_data *epd)