Fix a LOR between the NFS server and server side krpc.

Recent testing of the NFS-over-TLS code found a LOR between the mutex lock
used for sessions and the sleep lock used for server side krpc socket
structures in nfsrv_checksequence(). This was fixed by r365789.
A similar bug exists in nfsrv_bindconnsess(), where SVC_RELEASE() is called
while mutexes are held.
This patch applies a fix similar to r365789, moving the SVC_RELEASE() call
down to after the mutexes are released.

This patch fixes the problem by moving the SVC_RELEASE() call in
nfsrv_checksequence() down a few lines to below where the mutex is released.

MFC after: 1 week