Avoid issuing spa config updates for physical path when not necessary

ZFS's configuration needs to be updated whenever the physical path for a
device changes, but not when a new device is introduced. This is because new
devices necessarily cause config updates, but only if they are actually
accepted into the pool.

sys/cddl/contrib/opensolaris/uts/common/fs/zfs/vdev_geom.c
Split vdev_geom_set_physpath out of vdev_geom_attrchanged. When
setting the vdev's physical path, only request a config update if
the physical path has changed. Don't request it when opening a
device for the first time, because the config sync will happen
anyway upstack.

sys/geom/geom_dev.c
Split g_dev_set_physpath and g_dev_set_media out of
g_dev_attrchanged

Submitted by: will, asomers
MFC after: 4 weeks
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D6428