_stp_get_mm_counter: Fix kernel version check.

_stp_get_mm_counter: Fix kernel version check.

The kernel version check was wrong in two ways. First it tested against
kernel_v >= "3.1.0" but linux commit 172703 "mm: delete non-atomic mm
counter implementation" was actually integrated for 3.0. But fedora
names their 3.0 kernels 2.6.40 (and 3.1 kernels 2.6.41, depending on
release). So change the test to kernel_v >= "2.6.40".

The reason this wasn't found earlier was that there is another check
for CONFIG_NR_CPUS >= CONFIG_SPLIT_PTLOCK_CPUS which is almost always
true (but not on arm tegra kernels).