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>
(cherry picked from commit
c4a342cec1ced80128f82758c7a2192b23f4017a)