once: fix section mismatch on clang builds

On older kernels (5.4 and older), building the kernel with clang can
cause the section name to end up with "" in them, which can cause lots
of runtime issues as that is not normally a valid portion of the string.

This was fixed up in newer kernels with commit 33def84 ("treewide:
Convert macro and uses of __section(foo) to __section("foo")") but
that's too heavy-handed for older kernels.

So for now, fix up the problem that commit 62c0798 ("once: add
DO_ONCE_SLOW() for sleepable contexts") caused by being backported by
removing the "" characters from the section definition.

Reported-by: Oleksandr Tymoshenko <ovt@google.com>
Reported-by: Yongqin Liu <yongqin.liu@linaro.org>
Tested-by: Yongqin Liu <yongqin.liu@linaro.org>
Cc: Naresh Kamboju <naresh.kamboju@linaro.org>
Link: https://lore.kernel.org/r/20221029011211.4049810-1-ovt@google.com
Link: https://lore.kernel.org/r/CAMSo37XApZ_F5nSQYWFsSqKdMv_gBpfdKG3KN1TDB+QNXqSh0A@mail.gmail.com
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Willy Tarreau <w@1wt.eu>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: David S. Miller <davem@davemloft.net>
Cc: Sasha Levin <sashal@kernel.org>
Cc: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/once.h

@@ -64,7 +64,7 @@ void __do_once_slow_done(bool *done, struct static_key_true *once_key,
 #define DO_ONCE_SLOW(func, ...)						     \
 	({								     \
 		bool ___ret = false;					     \
-		static bool __section(".data.once") ___done = false;	     \
+		static bool __section(.data.once) ___done = false;	     \
 		static DEFINE_STATIC_KEY_TRUE(___once_key);		     \
 		if (static_branch_unlikely(&___once_key)) {		     \
 			___ret = __do_once_slow_start(&___done);	     \