zfsd(8): Close a race condition when onlining a disk paritition

When inserting a partitioned disk, devfs and geom will announce the whole
disk before they announce the partition. If the partition containing ZFS
extends to one of the disk's extents, then zfsd will see a ZFS label on the
whole disk and attempt to online it. ZFS is smart enough to activate the
partition instead of the whole disk, but only if GEOM has already created
the partition's provider.

cddl/contrib/opensolaris/lib/libzfs/common/libzfs.h
cddl/contrib/opensolaris/lib/libzfs/common/libzfs_import.c
Add a zpool_read_all_labels method. It's similar to
zpool_read_label, but it will return the number of labels found.

cddl/usr.sbin/zfsd/zfsd_event.cc
When processing a DevFS CREATE event, only online a VDEV if we can
read all four ZFS labels.

Reviewed by: mav
MFC after: 3 weeks
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D11920