srp_transport: Fix attribute registration

Register transport attributes after the attribute array has been set
up instead of before.  The current code can trigger a race condition
because the code reading the attribute array can run on another thread
than the code that initialized that array.  Make sure that any code
reading the attribute array will see all values written into that
array.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Cc: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: Robert Jennings <rcj@linux.vnet.ibm.com>
Acked-by: David Dillow <dillowda@ornl.gov>
Signed-off-by: Roland Dreier <roland@purestorage.com>