staging: lustre: selftest: fix _post_buffer context imbalance

Fix the sparse warning about a context imbalance in the
srpc_service_post_buffer function by telling sparse that it
should expect the lock to be held on entry and held on exit.

  drivers/staging/lustre/lnet/selftest/rpc.c:508:17: warning: context imbalance in 'srpc_service_post_buffer' - unexpected unlock

Signed-off-by: Jeremiah Mahler <jmmahler@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>