xen: execve's error paths don't pin the mm before unpinning

execve's error paths don't activate (and therefore pin) the mm before
calling exit_mmap to free it up, so don't try to unpin unless it is
actually pinned.  This prevents a BUG_ON from triggering.

Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Cc: Christian Ostheimer <osth@freesurf.ch>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>