Honour mpsafe_vfs=0 in softdep_flush.  This is basically a MFC of revision 1.206, except that Giant remains unconditionally acquired in the #ifdef QUOTA case here (as QUOTA-enabled UFS on RELENG_6 is not MPSAFE).
Honour mpsafe_vfs=0 in softdep_flush.  This is basically a MFC of revision
1.206, except that Giant remains unconditionally acquired in the #ifdef
QUOTA case here (as QUOTA-enabled UFS on RELENG_6 is not MPSAFE).

Reviewed by:	kib

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase