Add a min size parameter to btrfs_alloc_extent

On huge machines, delayed allocation may try to allocate massive extents.
This change allows btrfs_alloc_extent to return something smaller than
the caller asked for, and the data allocation routines will loop over
the allocations until it fills the whole delayed alloc.

Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 46f54df..7869c01 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -125,6 +125,7 @@
 	while(num_bytes > 0) {
 		cur_alloc_size = min(num_bytes, root->fs_info->max_extent);
 		ret = btrfs_alloc_extent(trans, root, cur_alloc_size,
+					 root->sectorsize,
 					 root->root_key.objectid,
 					 trans->transid,
 					 inode->i_ino, start, 0,
@@ -133,6 +134,7 @@
 			WARN_ON(1);
 			goto out;
 		}
+		cur_alloc_size = ins.offset;
 		ret = btrfs_insert_file_extent(trans, root, inode->i_ino,
 					       start, ins.objectid, ins.offset,
 					       ins.offset);