lsm: use lsm_blob_alloc() in lsm_bdev_alloc()

Convert the lsm_bdev_alloc() function to use the lsm_blob_alloc() helper
like all of the other LSM security blob allocators.

Reviewed-by: Casey Schaufler <casey@schaufler-ca.com>
Acked-by: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>