erofs: make the size of read data stored in buffer_ofs

Since now xattr_iter structures have been unified, make the size of the
read data stored in buffer_ofs.  Don't bother reusing buffer_size for
this use, which may be confusing.

This is in preparation for the following further cleanup.

Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20230613074114.120115-4-jefflexu@linux.alibaba.com
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

Author: lostjeffle

fs/erofs/xattr.c

@@ -315,7 +315,7 @@ static int xattr_checkbuffer(struct erofs_xattr_iter *it,
 {
 	int err = it->buffer_size < value_sz ? -ERANGE : 0;
 
-	it->buffer_size = value_sz;
+	it->buffer_ofs = value_sz;
 	return !it->buffer ? 1 : err;
 }
 
@@ -348,7 +348,7 @@ static int inline_getxattr(struct inode *inode, struct erofs_xattr_iter *it)
 		if (ret != -ENOATTR)
 			break;
 	}
-	return ret ? ret : it->buffer_size;
+	return ret ? ret : it->buffer_ofs;
 }
 
 static int shared_getxattr(struct inode *inode, struct erofs_xattr_iter *it)
@@ -371,7 +371,7 @@ static int shared_getxattr(struct inode *inode, struct erofs_xattr_iter *it)
 		if (ret != -ENOATTR)
 			break;
 	}
-	return ret ? ret : it->buffer_size;
+	return ret ? ret : it->buffer_ofs;
 }
 
 static bool erofs_xattr_user_list(struct dentry *dentry)