xfs: annotate struct xfs_attr_list_context with __counted_by_ptr

Add the `__counted_by_ptr` attribute to the `buffer` field of `struct
xfs_attr_list_context`. This field is used to point to a buffer of
size `bufsize`.

The `buffer` field is assigned in:
1. `xfs_ioc_attr_list` in `fs/xfs/xfs_handle.c`
2. `xfs_xattr_list` in `fs/xfs/xfs_xattr.c`
3. `xfs_getparents` in `fs/xfs/xfs_handle.c` (implicitly initialized to NULL)

In `xfs_ioc_attr_list`, `buffer` was assigned before `bufsize`. Reorder
them to ensure `bufsize` is set before `buffer` is assigned, although
no access happens between them.

In `xfs_xattr_list`, `buffer` was assigned before `bufsize`. Reorder
them to ensure `bufsize` is set before `buffer` is assigned.

In `xfs_getparents`, `buffer` is NULL (from zero initialization) and
remains NULL. `bufsize` is set to a non-zero value, but since `buffer`
is NULL, no access occurs.

In all cases, the pointer `buffer` is not accessed before `bufsize` is set.

This patch was generated by CodeMender and reviewed by Bill Wendling.
Tested by running xfstests.

Signed-off-by: Bill Wendling <morbo@google.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>

fs/xfs/libxfs/xfs_attr.h

@@ -55,7 +55,8 @@ struct xfs_attr_list_context {
 	struct xfs_trans	*tp;
 	struct xfs_inode	*dp;		/* inode */
 	struct xfs_attrlist_cursor_kern cursor;	/* position in list */
-	void			*buffer;	/* output buffer */
+	/* output buffer */
+	void			*buffer __counted_by_ptr(bufsize);
 
 	/*
 	 * Abort attribute list iteration if non-zero.  Can be used to pass

fs/xfs/xfs_handle.c

@@ -443,8 +443,8 @@ xfs_ioc_attr_list(
 	context.dp = dp;
 	context.resynch = 1;
 	context.attr_filter = xfs_attr_filter(flags);
-	context.buffer = buffer;
 	context.bufsize = round_down(bufsize, sizeof(uint32_t));
+	context.buffer = buffer;
 	context.firstu = context.bufsize;
 	context.put_listent = xfs_ioc_attr_put_listent;
 

fs/xfs/xfs_xattr.c

@@ -332,8 +332,8 @@ xfs_vn_listxattr(
 	memset(&context, 0, sizeof(context));
 	context.dp = XFS_I(inode);
 	context.resynch = 1;
-	context.buffer = size ? data : NULL;
 	context.bufsize = size;
+	context.buffer = size ? data : NULL;
 	context.firstu = context.bufsize;
 	context.put_listent = xfs_xattr_put_listent;