x86/ioperm: Prevent NULL dereference on theoretical missing IO bitmap

Outside the IOPL emulation path, the IO bitmap is always expected
to be allocated when TIF_IO_BITMAP is set. The paranoid WARN_ON_ONCE()
handles the case where the flag and the pointer got out of sync.
In this theoretical scenario, which presumes some other bug in the
code that triggers the WARN_ON_ONCe(), return early, instead of
continuing and dereferencing a NULL pointer.

[ mingo: Clarified the changelog. ]

Signed-off-by: Li RongQing <lirongqing@baidu.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Sohil Mehta <sohil.mehta@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Link: https://patch.msgid.link/20260615070115.4720-1-lirongqing@baidu.com

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