libpod: simplify resolveWorkDir()

libpod: simplify resolveWorkDir()

The code checks for isPathOnVolume and isPathOnMount so we can just use
the SecureJoin here directly to check for path existance.

Then instead of walking symlinks and trying to guess if they are on a
mount just assume if it is a link (path is different from the normal
joined one) then don't error out early and let the OCI runtime deal with
it. The runtime does produce a less readable error but it still fails
and we have much less fragile code.

Signed-off-by: Paul Holzinger <pholzing@redhat.com>