rseq: Mark rseq_arm_slice_extension_timer() __always_inline

objtool warns about this function being called inside of a uaccess
section:

kernel/entry/common.o: warning: objtool: irqentry_exit+0x1dc: call to rseq_arm_slice_extension_timer() with UACCESS enabled

Interestingly, this happens with CONFIG_RSEQ_SLICE_EXTENSION disabled,
so this is an empty function, as the normal implementation is
already marked __always_inline.

I could reproduce this multiple times with gcc-11 but not with gcc-15,
so the compiler probably got better at identifying the trivial function.

Mark all the empty helpers for !RSEQ_SLICE_EXTENSION as __always_inline
for consistency, avoiding this warning.

Fixes: 0ac3b5c ("rseq: Implement time slice extension enforcement timer")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260206074122.709580-1-arnd@kernel.org

Author: arndb

include/linux/rseq_entry.h

@@ -216,10 +216,10 @@ static __always_inline bool rseq_grant_slice_extension(bool work_pending)
 }
 
 #else /* CONFIG_RSEQ_SLICE_EXTENSION */
-static inline bool rseq_slice_extension_enabled(void) { return false; }
-static inline bool rseq_arm_slice_extension_timer(void) { return false; }
-static inline void rseq_slice_clear_grant(struct task_struct *t) { }
-static inline bool rseq_grant_slice_extension(bool work_pending) { return false; }
+static __always_inline bool rseq_slice_extension_enabled(void) { return false; }
+static __always_inline bool rseq_arm_slice_extension_timer(void) { return false; }
+static __always_inline void rseq_slice_clear_grant(struct task_struct *t) { }
+static __always_inline bool rseq_grant_slice_extension(bool work_pending) { return false; }
 #endif /* !CONFIG_RSEQ_SLICE_EXTENSION */
 
 bool rseq_debug_update_user_cs(struct task_struct *t, struct pt_regs *regs, unsigned long csaddr);