drivers: uio_dmem_genirq: Allow partial success when opening device

The uio device should not fail on open just because one memory allocation
fails. The device might export several regions, the failure of some of
which may or may not be a problem for the user space driver.  Failing
regions will remain unmapped, and successful regions will be mapped and
exported to user space.  Also deals with the case where failing to map
a region after successfully allocating others would not unmap the
successfully allocated regions before dying.

Signed-off-by: Damian Hobson-Garcia <dhobsong@igel.co.jp>
Cc: "Hans J. Koch" <hjk@hansjkoch.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>