xenbus: fix deadlock on writes to /proc/xen/xenbus

xenbus: fix deadlock on writes to /proc/xen/xenbus

/proc/xen/xenbus does not work correctly.  A read blocked waiting for
a xenstore message holds the mutex needed for atomic file position
updates.  This blocks any writes on the same file handle, which can
deadlock if the write is needed to unblock the read.

Clear FMODE_ATOMIC_POS when opening this device to always get
character device like sematics.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Orabug: 25425387
(cherry picked from commit 581d21a2d02a798ee34e56dbfa13f891b3a90c30)
Jira: OCC-36718
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Chuck Anderson <chuck.anderson@oracle.com>