mtip32xx: Fix for rmmod crash when drive is in FTL rebuild

mtip32xx: Fix for rmmod crash when drive is in FTL rebuild

BugLink: http://bugs.launchpad.net/bugs/1572722
commit 59cf70e236c96594d9f1e065755d8fce9df5356b upstream.

When FTL rebuild is in progress, alloc_disk() initializes the disk
but device node will be created by add_disk() only after successful
completion of FTL rebuild. So, skip deletion of device node in
removal path when FTL rebuild is in progress.

Signed-off-by: Selvan Mani <smani@micron.com>
Signed-off-by: Asai Thambi S P <asamymuthupa@micron.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>