VMCI: Use 32bit atomics for queue headers on X86_32

This change restricts the reading and setting of the head and tail
pointers on 32bit X86 to 32bit for both correctness and
performance reasons. On uniprocessor X86_32, the atomic64_read
may be implemented as a non-locked cmpxchg8b. This may result in
updates to the pointers done by the VMCI device being overwritten.
On MP systems, there is no such correctness issue, but using 32bit
atomics avoids the overhead of the locked 64bit operation. All this
is safe because the queue size on 32bit systems will never exceed
a 32bit value.

Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Jorgen Hansen <jhansen@vmware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>