Instead of forcing vn_start_write() to reset mp back to NULL for the failed calls with non-NULL vp, explicitely clear mp after failure.
Instead of forcing vn_start_write() to reset mp back to NULL for the
failed calls with non-NULL vp, explicitely clear mp after failure.

Tested by:	stass
Reviewed by:	tegge
PR:		123768
MFC after:	1 week

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