i386: Fix broken CONFIG_COMPAT_VDSO on i386

i386: Fix broken CONFIG_COMPAT_VDSO on i386

After updating several machines to 2.6.20, I can't boot  anymore the single
one of them that supports the NX bit and is configured as a 32-bit system.

My understanding is that the VDSO changes in 2.6.20-rc7 were not fully
cooked, in that with that config option enabled VDSO_SYM(x) now equals
x, meaning that an address in the fixmap area is now being passed to
apps via AT_SYSINFO. However, the page is mapped with PAGE_READONLY
rather than PAGE_READONLY_EXEC.

I'm not certain whether having app code go through the fixmap area is
intended, but in case it is here is the simple patch that makes things work
again.

Cc: Theodore Tso <tytso@mit.edu>
Signed-off-by: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>