rcu: Mark lockdep_assert_rcu_helper() __always_inline

rcu: Mark lockdep_assert_rcu_helper() __always_inline

There are some configurations in which lockdep_assert_rcu_helper() ends up
not being inlined, for some reason. This leads to a link failure because
now the caller tries to pass a nonexistant __ctx_lock_RCU structure:

ld: lib/test_context-analysis.o: in function `test_rcu_assert_variants':
test_context-analysis.c:(.text+0x275c): undefined reference to `RCU'
ld: test_context-analysis.c:(.text+0x276c): undefined reference to `RCU_BH'
ld: test_context-analysis.c:(.text+0x2774): undefined reference to `RCU_SCHED'

I saw this in one out of many 32-bit arm builds using gcc-15.2, but
it probably happens in others as well.

Mark this function as __always_inline to fix the build.

Fixes: fe00f6e84621 ("rcu: Support Clang's context analysis")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Link: https://patch.msgid.link/20260202095507.1237440-1-arnd@kernel.org