xfs_db: fix a complaint about a printf buffer overrun

xfs_db: fix a complaint about a printf buffer overrun

gcc 11 warns that stack_f doesn't allocate a sufficiently large buffer
to hold the printf output.  I don't think the io cursor stack is really
going to grow to 4 billion levels deep, but let's fix this anyway.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>