Allow partial extent motion

Refilling a client's data_avail is the only alloc_move call that doesn't
try and limit the number of blocks that it dirties.  If it doesn't find
sufficiently large extents it can exhaust the server's alloc budget
without hitting the target.  It'll try to dirty blocks and return a hard
error.

This changes that behaviour to allow returning 0 if it moved any
extents.  Other callers can deal with partial progress as they already
limit the blocks they dirty.  This will also return ENOSPC if it hadn't
moved anything just as the current code would.

The result is that data fill can not necessarily hit the target.  It
might take multiple commits to fill the data_avail btree.

Signed-off-by: Zach Brown <zab@versity.com>

kmod/src/alloc.c

@@ -976,6 +976,16 @@ int scoutfs_alloc_move(struct super_block *sb, struct scoutfs_alloc *alloc,
 			break;
 		}
 
+		/* return partial if the server alloc can't dirty any more */
+		if (scoutfs_alloc_meta_low(sb, alloc, 50 + extent_mod_blocks(src->root.height) +
+						      extent_mod_blocks(dst->root.height))) {
+			if (WARN_ON_ONCE(!moved))
+				ret = -ENOSPC;
+			else
+				ret = 0;
+			break;
+		}
+
 		/* searching set start/len, finish initializing alloced extent */
 		ext.map = found.map ? ext.start - found.start + found.map : 0;
 		ext.flags = found.flags;