msg/Pipe: do not hold pipe_lock for verify_authorizer()

We shouldn't hold the pipe_lock while doing the ms_verify_authorizer
upcalls.

Fix by unlocking a bit earlier, and verifying our state is still correct
in the failure path.

This regression was introduced by ecab4bb9513385bd765cca23e4e2fadb7ac4bac2.

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>