rx: hold call->lock across RXS_PreparePacket

rx: hold call->lock across RXS_PreparePacket

RX Security Classes have a right to assume that when RXS_PreparePacket
is called that they have exclusive access to the rx_call structure.
Commit e445faa68c5ec6e47d3fd9d7318ade71d98703a9 unintentionally
failed to acquire the call->lock prior to RXS_PreparePacket being
called.

Reviewed-on: http://gerrit.openafs.org/6904
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Alistair Ferguson <alistair.ferguson@mac.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Tested-by: Jeffrey Altman <jaltman@secure-endpoints.com>
(cherry picked from commit 0fdcf171a8d2b2153648b6b799096e09eb469beb)

Change-Id: I184a3c71418eb78742fbf1bf609daca1aea57f26
Reviewed-on: http://gerrit.openafs.org/6909
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>