runtime-fix #2: route __storeload_barrier to userland path under FSTACK

amd64 __storeload_barrier() uses `lock; addl $0,%gs:OFFSETOF_MONITORBUF`
when _KERNEL is defined, which accesses the per-CPU segment via the %gs
selector. That selector is set up by the FreeBSD kernel for in-kernel
threads, but never initialized for userland processes — so when F-Stack
runs the same instruction it dereferences an unmapped address and the
process dies with SIGSEGV.

After fix #1 unblocked uma_startup1, the next crash surfaced in
smr_create() (PC at +150 -> `lock addl $0x0,%gs:0x100`), with the stack:

    smr_create -> zone_ctor -> uma_zcreate -> filelistinit -> mi_startup

The userland branch of __storeload_barrier already exists (uses
`lock; addl $0,-8(%rsp)` against the redzone, which is safe and equally
serializing on x86). Just include FSTACK in the predicate that selects
the userland implementation:

    #if defined(_KERNEL) && !defined(FSTACK)
        ... %gs-based variant ...
    #else /* !_KERNEL || FSTACK */
        ... rsp-based variant ...
    #endif

Verified by disassembling libfstack.ro::smr_create after a clean rebuild:
the instruction is now `lock addl $0x0,-0x8(%rsp)` instead of
`lock addl $0x0,%gs:0x100`.

Author: jfb8856606

freebsd/amd64/include/atomic.h

@@ -322,12 +322,18 @@ atomic_testandclear_long(volatile u_long *p, u_int v)
 static __inline void
 __storeload_barrier(void)
 {
-#if defined(_KERNEL)
+#if defined(_KERNEL) && !defined(FSTACK)
 	__asm __volatile("lock; addl $0,%%gs:%c0"
 	    : : "i" (OFFSETOF_MONITORBUF) : "memory", "cc");
-#else /* !_KERNEL */
+#else /* !_KERNEL || FSTACK */
+	/*
+	 * F-Stack DP-RT-2026-06-01: F-Stack runs in user space; %gs PCPU
+	 * segment is not set up by the userland process, so accessing
+	 * %gs:OFFSETOF_MONITORBUF would SIGSEGV (observed in smr_create()).
+	 * Fall back to the userland-safe rsp variant.
+	 */
 	__asm __volatile("lock; addl $0,-8(%%rsp)" : : : "memory", "cc");
-#endif /* _KERNEL*/
+#endif /* _KERNEL && !FSTACK */
 }
 
 #define	ATOMIC_LOAD(TYPE)					\