fix the following 'make includecheck' warning:

  drivers/xen/evtchn.c: linux/errno.h is included more than once.

Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>
Cc:  chrisw@sous-sol.org
Cc: Sam Ravnborg <sam@ravnborg.org>
LKML-Reference: <1247067749.4382.90.camel@ht.satnam>
Acked-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

Define a new per_user_data mutex to serialize bind/unbind operations
to prevent them from racing with each other.  Fix error returns
and don't do a bind while holding a spinlock.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

This driver is used by application which wish to receive notifications
from the hypervisor or other guests via Xen's event channel
mechanism. In particular it is used by the xenstore daemon in domain
0.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>