IncRemove: Set "auto-read" only after successful excl open.

IncRemove: Set "auto-read" only after successful excl open.

"mdadm -If" - triggered from udev rules when disk is removed from OS -
tries to set array in auto-read-only mode. This can interrupt rebuild
process which is started automatically, e.g. if array is mounted and
spare disk is available (I/O error is detected faster than removing
failed disk by mdadm).
This patch prevents "mdadm -If" from setting array into "auto-read-only",
by requiring exclusive open to succeed.

Signed-off-by: Pawel Baldysiak <pawel.baldysiak@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>