Fix incomplete freed list head recovery in dirty_alloc_blocks

dirty_alloc_blocks saves the freed list state before modifying it so
it can roll back on error.  But it only saved the block_ref (blkno
+ seq), not the full alloc_list_head which also includes first_nr,
total_nr, and flags.

When the freed list's head block is nearly full, dirty_alloc_blocks
forces allocation of a new empty head block. It zeros alloc->freed.ref
and sets alloc->freed.first_nr = 0.  dirty_list_block then sees the
empty ref, allocates a fresh block, and updates alloc->freed.ref to
point at it. first_nr = 0 correctly describes that new empty block. All
good so far.

If the subsequent avail dirty_list_block fails, the error path restores
alloc->freed.ref to orig_freed but leaves alloc->freed.first_nr at 0.
The original head block still holds its N entries on disk, but the
in-memory head now claims first_nr = 0 -- the recovery should have
rewound first_nr back to N along with the ref, but had no saved copy
to rewind from.

On the next dirty_alloc_blocks call the threshold check sees first_nr =
0! (full empty space) and skips the new-block path.  dirty_list_block
CoWs the existing head block and list_block_add writes new entries
past the N already-present blknos while incrementing first_nr from 0.
The head's first_nr drifts permanently below the block's actual nr.
This leads to the alloc list head/block mismatch BUG_ON at alloc.c:375
and downstream extent overlap errors that permanently stall the server.

Fix by saving and restoring the full scoutfs_alloc_list_head struct
instead of just the scoutfs_block_ref. Exposed by stress testing.

Signed-off-by: Auke Kok <auke.kok@versity.com>

kmod/src/alloc.c

@@ -491,7 +491,7 @@ static int dirty_alloc_blocks(struct super_block *sb,
 			      struct scoutfs_alloc *alloc,
 			      struct scoutfs_block_writer *wri)
 {
-	struct scoutfs_block_ref orig_freed;
+	struct scoutfs_alloc_list_head orig_freed;
 	struct scoutfs_alloc_list_block *lblk;
 	struct scoutfs_block *av_bl = NULL;
 	struct scoutfs_block *fr_bl = NULL;
@@ -509,7 +509,7 @@ static int dirty_alloc_blocks(struct super_block *sb,
 	mutex_lock(&alloc->mutex);
 
 	/* undo dirty freed if we get an error after */
-	orig_freed = alloc->freed.ref;
+	orig_freed = alloc->freed;
 
 	if (alloc->dirty_avail_bl != NULL) {
 		ret = 0;
@@ -550,7 +550,7 @@ static int dirty_alloc_blocks(struct super_block *sb,
 	if (link_orig) {
 		/* .. and point the new block at the rest of the list */
 		lblk = fr_bl->data;
-		lblk->next = orig_freed;
+		lblk->next = orig_freed.ref;
 		lblk = NULL;
 	}
 
@@ -575,10 +575,10 @@ static int dirty_alloc_blocks(struct super_block *sb,
 	ret = 0;
 
 out:
-	if (ret < 0 && alloc->freed.ref.blkno != orig_freed.blkno) {
+	if (ret < 0 && alloc->freed.ref.blkno != orig_freed.ref.blkno) {
 		if (fr_bl)
 			scoutfs_block_writer_forget(sb, wri, fr_bl);
-		alloc->freed.ref = orig_freed;
+		alloc->freed = orig_freed;
 	}
 
 	mutex_unlock(&alloc->mutex);