mkfs: fix PATH_MAX check

Per the snprintf manpage, the correct means to check snprintf for an
insufficiently large buffer is to check if its return value is >= @size,
not > @size.  Codex found this.

Cc: <linux-xfs@vger.kernel.org> # v6.17.0
Fixes: 8a4ea72724930c ("proto: add ability to populate a filesystem from a directory")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>