x86/itmt: Use guard() for itmt_update_mutex

Use guard() for itmt_update_mutex which avoids the extra mutex_unlock()
in the bailout and return paths.

Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Tim Chen <tim.c.chen@linux.intel.com>
Link: https://lore.kernel.org/r/20241223043407.1611-3-kprateek.nayak@amd.com