fs: gracefully handle ->get_block not mapping bh in __mpage_writepage

fs: gracefully handle ->get_block not mapping bh in __mpage_writepage

When filesystem's ->get_block function does not map the buffer head when
called from __mpage_writepage(), __mpage_writepage() will happily go and
pass bogus bdev and block number to bio allocation routines which leads to
crashes sooner or later.

E.g.  UDF can do this because it doesn't want to allocate blocks from
->writepages callbacks.  It allocates blocks on write or page fault but
writeback can still spot dirty buffers without underlying blocks allocated
e.g.  if blocksize < pagesize, the tail page is dirtied (which means all
its buffers are dirtied), and truncate extends the file so that some
buffer starts to be within i_size.

Link: https://lkml.kernel.org/r/20230126085155.26395-1-jack@suse.cz
Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>