ukernel: Fix AFS_GUNLOCK panic in rx_ServerProc

ukernel: Fix AFS_GUNLOCK panic in rx_ServerProc

At the beginning of UKERNEL's rx_ServerProc a call to AFS_GUNLOCK
panics with 'afs global lock not held'.

The commit 'afs: Drop GLOCK for various Rx calls' (d5e1428a3b) altered
afs_RXCallBackServer so the global lock is released prior to calling
rx_ServerProc, and to reacquire the lock after rx_ServerProc returns.

Remove the AFS_GUNLOCK at the start and the AFS_GLOCK at the end of
UKERNEL's rx_ServerProc.

Change-Id: Id0e2fef574e4d3473e83f77615d194448f55ab47
Reviewed-on: https://gerrit.openafs.org/14963
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>