Fix NULL pointer dereference in bl_free_device().

Fix NULL pointer dereference in bl_free_device().

When bl_parse_deviceid() fails in bl_alloc_deviceid_node() on
blkdev_get_by_*() step we get an pnfs_block_dev struct that is
uninitialized except for bdev field which is set to whatever error
blkdev_get_by_*() returns.  bl_free_device() then tries to call
blkdev_put() if bdev is not 0 resulting in a wrong pointer dereference.

Fixing this by setting bdev in struct pnfs_block_dev only if we didn't
get an error from blkdev_get_by_*().

Signed-off-by: Artem Savkov <asavkov@redhat.com>
Reviewed-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>