oom: log one-time warning if kernel doesn't provide memory.swap.current

The kernel can be compiled without support for any memory.swap.* files, or
it can be disabled at boot time with the 'swapaccount=0' boot parameter,
so if the file doesn't exist log warning indicating the kernel doesn't
support the file and the user may need to try using the 'swapaccount=1'
boot param.

Note that the actual error from the call to fopen() is ENOENT, but
that is translated into ENODATA in cg_get_attribute_as_uint64()