bcachefs: Ensure journal space is block size aligned

We don't require that bucket size is block size aligned (although it
should be!) - so we need to handle this in the journal code.

This fixes an assertion pop in jorunal_entry_close(), where the journal
entry overruns available space - after rounding it up to block size.

Fixes: https://github.com/koverstreet/bcachefs/issues/854
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/journal_reclaim.c

@@ -252,7 +252,10 @@ void bch2_journal_space_available(struct journal *j)
 
 	bch2_journal_set_watermark(j);
 out:
-	j->cur_entry_sectors	= !ret ? j->space[journal_space_discarded].next_entry : 0;
+	j->cur_entry_sectors	= !ret
+		? round_down(j->space[journal_space_discarded].next_entry,
+			     block_sectors(c))
+		: 0;
 	j->cur_entry_error	= ret;
 
 	if (!ret)