Linux 2.6.38 compat, blkdev_get_by_path()

Linux 2.6.38 compat, blkdev_get_by_path()

The open_bdev_exclusive() function has been replaced (again) by the
more generic blkdev_get_by_path() function.  Additionally, the
counterpart function close_bdev_exclusive() has been replaced by
blkdev_put().  Because these functions are more generic versions
of the functions they replaced the compatibility macro must add
the FMODE_EXCL mask to ensure they are exclusive.

Closes #114