vdso/gettimeofday: Force inlining of __cvdso_clock_getres_common()

With CONFIG_CC_OPTIMIZE_FOR_SIZE=y, GCC may decide not to inline
__cvdso_clock_getres_common(). This introduces spurious internal
function calls in the vDSO fastpath.

Furthermore, with automatic stack variable initialization
(CONFIG_INIT_STACK_ALL_ZERO or CONFIG_INIT_STACK_ALL_PATTERN) GCC can emit
a call to memset() which is not valid in the vDSO.

Mark __cvdso_clock_getres_common() as __always_inline to avoid both issues.

Paradoxically the inlining even reduces the size of the code:

$ ./scripts/bloat-o-meter arch/powerpc/kernel/vdso/vgettimeofday-32.o.before arch/powerpc/kernel/vdso/vgettimeofday-32.o.after
add/remove: 0/1 grow/shrink: 1/1 up/down: 52/-148 (-96)
Function                                     old     new   delta
__c_kernel_clock_getres_time64                92     144     +52
__c_kernel_clock_getres                      136     132      -4
__cvdso_clock_getres_common                  144       -    -144
Total: Before=2788, After=2692, chg -3.44%

With CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE=y the functions are always inlined
and therefore the behaviour stays the same.

See also the equivalent change for clock_gettime() in commit b91c8c4
("lib/vdso: Force inlining of __cvdso_clock_gettime_common()").

Fixes: 21bbfd7 ("x86/vdso: Provide clock_getres_time64() for x86-32")
Fixes: 1149dcd ("ARM: VDSO: Provide clock_getres_time64()")
Fixes: f10c2e7 ("arm64: vdso32: Provide clock_getres_time64()")
Fixes: bec06cd ("MIPS: vdso: Provide getres_time64() for 32-bit ABIs")
Fixes: 759a1f9 ("powerpc/vdso: Provide clock_getres_time64()")
Reported-by: Sverdlin, Alexander <alexander.sverdlin@siemens.com>
Suggested-by: Christophe Leroy (CS GROUP) <chleroy@kernel.org>
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Link: https://lore.kernel.org/lkml/230c749f-ebd6-4829-93ee-601d88000a45@kernel.org/
Link: https://patch.msgid.link/20260123-vdso-clock_getres-inline-v1-1-4d6203b90cd3@linutronix.de
Closes: https://lore.kernel.org/lkml/f45316f65a46da638b3c6aa69effd8980e6677b9.camel@siemens.com/

Author: t-8ch

lib/vdso/gettimeofday.c

@@ -421,7 +421,7 @@ static __maybe_unused __kernel_old_time_t __cvdso_time(__kernel_old_time_t *time
 #endif /* VDSO_HAS_TIME */
 
 #ifdef VDSO_HAS_CLOCK_GETRES
-static __maybe_unused
+static __always_inline
 bool __cvdso_clock_getres_common(const struct vdso_time_data *vd, clockid_t clock,
 				 struct __kernel_timespec *res)
 {