If the L3 disable slot is already in use, return -EEXIST instead of
-EINVAL. The caller, store_cache_disable(), checks this return value to
print an appropriate warning.

Also, we want to signal with -EEXIST that the current index we're
disabling has actually been already disabled on the node:

$ echo 12 > /sys/devices/system/cpu/cpu3/cache/index3/cache_disable_0
$ echo 12 > /sys/devices/system/cpu/cpu3/cache/index3/cache_disable_0
-bash: echo: write error: File exists
$ echo 12 > /sys/devices/system/cpu/cpu3/cache/index3/cache_disable_1
-bash: echo: write error: File exists
$ echo 12 > /sys/devices/system/cpu/cpu5/cache/index3/cache_disable_1
-bash: echo: write error: File exists

The old code would say

-bash: echo: write error: Invalid argument

for disable slot 1 when playing the example above with no output in
dmesg, which is clearly misleading.
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/20120419070053.GB16645@elgon.mountain
[Boris: add testing for the other index too]
Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>