x86, 32-bit: ifdef out struct thread_struct::fs

After commit 464d1a78 aka
"[PATCH] i386: Convert i386 PDA code to use %fs"
%fs saved during context switch moved from thread_struct to pt_regs
and value on thread_struct became unused.

[ Impact: reduce thread_struct size on 32-bit ]

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Cc: containers@lists.linux-foundation.org
LKML-Reference: <20090503232952.GI16631@x200.localdomain>
Signed-off-by: Ingo Molnar <mingo@elte.hu>