cleanup: Optimize guards

cleanup: Optimize guards

Andrew reported that a guard() conversion of zone_lock increased the
code size unnecessarily.

It turns out the unconditional __GUARD_IS_ERR() is to blame. As
explored earlier [1], __GUARD_IS_ERR(), similar to IS_ERR_OR_NULL(),
generates somewhat sub-optimal code.

However, looking at things again, it is possible to avoid doing the
__GUARD_IS_ERR() unconditionally. Revert the normal destructors to a
simple NULL test and only add the IS_ERR bit to COND guards.

This cures the reported overhead; as compiled by GCC-16:

	page_alloc.o:

pre:	Total: Before=45299, After=45371, chg +0.16%
post:	Total: Before=45299, After=45026, chg -0.60%

[1] https://lkml.kernel.org/r/20250513085001.GC25891@noisy.programming.kicks-ass.net

Reported-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Dan Williams <dan.j.williams@intel.com>
Link: https://patch.msgid.link/20260309164516.GE606826@noisy.programming.kicks-ass.net