buffer: fix unintended successful return

If try_to_free_buffers() succeeded and then folio_alloc_buffers() failed,
grow_dev_folio() would return success.  This would be incorrect; memory
allocation failure is supposed to result in a failure.  It's a harmless
bug; the caller will simply go around the loop one more time and
grow_dev_folio() will correctly return a failure that time.  But it was an
unintended change and looks like a more serious bug than it is.

While I'm in here, improve the commentary about why we return success even
though we failed.

Link: https://lkml.kernel.org/r/20240101093848.2017115-1-willy@infradead.org
Fixes: 6d840a18

 ("buffer: return bool from grow_dev_folio()")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reported-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>