mm: assert the folio is locked in folio_start_writeback()

The folio must be locked when we start writeback in order to prevent
writeback from being started twice on the same folio.  I don't expect this
to catch any problems, but it should be good documentation.

Link: https://lkml.kernel.org/r/20250226153614.3774896-1-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>