x86/boot: Reject too long acpi_rsdp= values

x86/boot: Reject too long acpi_rsdp= values

cmdline_find_option() returns the full length of the parsed acpi_rsdp=
value. get_cmdline_acpi_rsdp() then silently truncates values which do
not fit in the val[] buffer.

Prevent boot_kstrtoul() from parsing a truncated value and then the
kernel from silently using the wrong RSDP address, see discussion in
Link:.

Issue a warning so that the user is aware that s/he supplied a malformed
value and can get feedback instead of silent crashes.

  [ bp: Make commit message more precise. ]

Fixes: 3c98e71b42a7 ("x86/boot: Add "acpi_rsdp=" early parsing")
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/all/20260617130417.36651-4-thorsten.blum@linux.dev