fortify: Fix incorrect reporting of read buffer size

When FORTIFY_SOURCE reports about a run-time buffer overread, the wrong
buffer size was being shown in the error message. (The bounds checking
was correct.)

Fixes: 3d965b33 ("fortify: Improve buffer overflow reporting")
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20250729231817.work.023-kees@kernel.org


Signed-off-by: Kees Cook <kees@kernel.org>