Avoid scanning all pvs in the system if operating on a device with mdas.

Avoid scanning all pvs in the system if operating on a device with mdas.

When we pv_read() a device that has an orphan vgname, we might need to scan
the system to be sure this is true.  However, if the PV has mdas, there's
no way possible for it to have an orphan vgname unless it is a true orphan.
Some areas of the code were optimized to take advantage of this fact, while
others were not (we would still do the expensive scan if a device had mdas
but had an orphan VG).

This patch unifies the code so that every place we are operating on such
a PV, we skip the expensive scan if there are mdas.

Signed-off-by: Dave Wysochanski <dwysocha@redhat.com>
Acked-by: Petr Rockai <prockai@redhat.com>
Acked-by: Alasdair G Kergon <agk@redhat.com>