x86/fred: Fix a build warning with allmodconfig due to 'inline' failing to inline properly

Change array_index_mask_nospec() to __always_inline because "inline" is
broken as https://www.kernel.org/doc/local/inline.html.

Fixes: 6786137bf8fd ("x86/fred: FRED entry/exit and dispatch code")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Xin Li (Intel) <xin@zytor.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20240202090225.322544-1-xin@zytor.com