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.

Change-Id: I5a9dfb2818fa17ba6fc1ab6f76ec5cde602c6f16
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>