mount.cifs: take extra care that mountpoint isn't changed during mount

mount.cifs: take extra care that mountpoint isn't changed during mount

It's possible to trick mount.cifs into mounting onto the wrong directory
by replacing the mountpoint with a symlink to a directory. mount.cifs
attempts to check the validity of the mountpoint, but there's still a
possible race between those checks and the mount(2) syscall.

To guard against this, chdir to the mountpoint very early, and only deal
with it as "." from then on out.

Signed-off-by: Jeff Layton <jlayton@redhat.com>