x86/mtrr: Remove unnecessary strlen() in mtrr_write()

The local variable length already holds the string length after calling
strncpy_from_user(). Using another local variable linlen and calling
strlen() is therefore unnecessary and can be removed. Remove linlen
and strlen() and use length instead.

No change in functionality intended.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20250225131621.329699-2-thorsten.blum@linux.dev