x86/tls: Clean up 'sel' variable usage in do_set_thread_area()

x86/tls: Clean up 'sel' variable usage in do_set_thread_area()

The top-level 'sel' variable in do_set_thread_area() was previously
marked __maybe_unused, but it is now only needed locally when
updating the current task.

Remove the unused top-level declaration and introduce a local 'sel'
variable where it is actually used

No functional change intended.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://patch.msgid.link/20260330085938.67985-4-ubizjak@gmail.com