once_lite: Simplify condition handling and fix context analysis

When WARN_ON_ONCE() wraps a conditional lock acquisition (such as
down_write_trylock()) on architectures relying on DO_ONCE_LITE_IF()
(e.g. arm), Clang's context analysis (Thread Safety Analysis) failed
with a false positive:

  fs/ext2/xattr.c:825:6: error: rw_semaphore 'EXT2_I().xattr_sem' is not held on every path through here [-Werror,-Wthread-safety-analysis]
    825 |         if (WARN_ON_ONCE(!down_write_trylock(&EXT2_I(inode)->xattr_sem)))
        |             ^

This happens because DO_ONCE_LITE_IF() branches on __ONCE_LITE_IF()'s
return value (__ret_once), creating an intermediate branch merge point
where the lock may or may not be held depending on whether the once-flag
(__already_done) was already set. Because the merge branch condition is
__ret_once rather than the trylock predicate (__ret_do_once), Clang
cannot reconcile the lockset at the branch merge points.

Fix it by refactoring __ONCE_LITE_IF() into an unconditional
__ONCE_LITE() primitive and redefining __ONCE_LITE_IF(condition) as:

  (unlikely(condition) && __ONCE_LITE())

This simplifies the implementation, short-circuits evaluation so that
__ONCE_LITE() is not called when the condition is false, and ensures
that DO_ONCE_LITE_IF() only enters __ONCE_LITE() when __ret_do_once is
true.

Reported-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/all/20260903072759.GA1750084@ax162/
Signed-off-by: Marco Elver <elver@google.com>
Tested-by: Nathan Chancellor <nathan@kernel.org> # build
Link: https://patch.msgid.link/20260903101843.3462767-1-elver@google.com
Signed-off-by: Nathan Chancellor <nathan@kernel.org>

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