x86/mtrr: Use kstrtoul() in parse_mtrr_spare_reg()

x86/mtrr: Use kstrtoul() in parse_mtrr_spare_reg()

Replace the deprecated simple_strtoul()¹ with kstrtoul() for parsing the early
boot parameter mtrr_spare_reg_nr. simple_strtoul() silently sets
nr_mtrr_spare_reg to 0 for invalid input instead of rejecting it and keeping
the default value.

Return kstrtoul()'s retval directly to propagate parsing failures instead of
treating them as success. Also return -EINVAL when '=' is missing from the
boot parameter and 'arg' is NULL.

  ¹ https://www.kernel.org/doc/html/latest/process/deprecated.html#simple-strtol-simple-strtoll-simple-strtoul-simple-strtoull

  [ bp: Massage commit message. ]

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://patch.msgid.link/20260302135341.3473-2-thorsten.blum@linux.dev