hdf5_msb

Straggler from previous commit: when closing the VFD SWMR virtual file, clear
the lower virtual file's `exc_owner` field.  While I'm here, remove a
gratuitous assertion.

This is part of a changeset that helps us avoid creating multiple H5F_shared_t
for one file when virtual datasets are used with VFD SWMR.  The old code for
deduplicating VFD SWMR H5F_shared_t instances did not work correctly with VFD
SWMR, so we'd end up with multiple H5F_shared_t all active on the same file.