Fix getting total available memory without psutil installed

[vutny]

What does this PR do?

Fixes Salt crash on Linux without psutil Python module installed.

What issues does this PR fix or reference?

The issue was introduced by this PR: #34683

Previous Behavior

[root@salt /]# salt --version
Traceback (most recent call last):
  File "/usr/bin/salt", line 10, in <module>
    salt_main()
  File "/usr/lib/python2.7/site-packages/salt/scripts.py", line 462, in salt_main
    import salt.cli.salt
  File "/usr/lib/python2.7/site-packages/salt/cli/salt.py", line 9, in <module>
    import salt.utils.job
  File "/usr/lib/python2.7/site-packages/salt/utils/job.py", line 8, in <module>
    import salt.minion
  File "/usr/lib/python2.7/site-packages/salt/minion.py", line 78, in <module>
    import salt.client
  File "/usr/lib/python2.7/site-packages/salt/client/__init__.py", line 29, in <module>
    import salt.config
  File "/usr/lib/python2.7/site-packages/salt/config/__init__.py", line 92, in <module>
    _DFLT_IPC_WBUFFER = _gather_buffer_space() * .5
  File "/usr/lib/python2.7/site-packages/salt/config/__init__.py", line 84, in _gather_buffer_space
    os_data = salt.grains.core.os_data()
  File "/usr/lib/python2.7/site-packages/salt/grains/core.py", line 1386, in os_data
    grains.update(_linux_gpu_data())
  File "/usr/lib/python2.7/site-packages/salt/grains/core.py", line 154, in _linux_gpu_data
    if __opts__.get('enable_lspci', True) is False:
NameError: global name '__opts__' is not defined

__opts__ dictionary is not available because you're loading configuration right now, "chicken and the egg" problem appears.

But just for getting mem_totalGrain value you don't need full blown OS data, only to know what's the OS kernel (see _memdata function). This should be safe on all supported platforms.

New Behavior

Salt works.

Tests written?

No