Extend CONFIG_DEBUG_LOCK_ALLOC check

The CONFIG_DEBUG_LOCK_ALLOC check at configure time was added to
detect when mutex_lock() is defined as a GPL-only symbol.  However,
the check as written only inferred this from this configuration
setting, it never actually checked.  This change introduces that
missing check to prevent false positives.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Add vdev_id for JBOD-friendly udev aliases

vdev_id parses the file /etc/zfs/vdev_id.conf to map a physical path
in a storage topology to a channel name.  The channel name is combined
with a disk enclosure slot number to create an alias that reflects the
physical location of the drive.  This is particularly helpful when it
comes to tasks like replacing failed drives.  Slot numbers may also be
re-mapped in case the default numbering is unsatisfactory.  The drive
aliases will be created as symbolic links in /dev/disk/by-vdev.

The only currently supported topologies are sas_direct and sas_switch:

o  sas_direct - a channel is uniquely identified by a PCI slot and a
   HBA port

o  sas_switch - a channel is uniquely identified by a SAS switch port

A multipath mode is supported in which dm-mpath devices are handled by
examining the first running component disk, as reported by 'multipath
-l'.  In multipath mode the configuration file should contain a
channel definition with the same name for each path to a given
enclosure.

vdev_id can replace the existing zpool_id script on systems where the
storage topology conforms to sas_direct or sas_switch.  The script
could be extended to support other topologies as well.  The advantage
of vdev_id is that it is driven by a single static input file that can
be shared across multiple nodes having a common storage toplogy.
zpool_id, on the other hand, requires a unique /etc/zfs/zdev.conf per
node and a separate slot-mapping file.  However, zpool_id provides the
flexibility of using any device names that show up in
/dev/disk/by-path, so it may still be needed on some systems.

vdev_id's functionality subsumes that of the sas_switch_id script, and
it is unlikely that anyone is using it, so sas_switch_id is removed.

Finally, /dev/disk/by-vdev is added to the list of directories that
'zpool import' will scan.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #713
Add /dev/mapper/ to search path

When creating pools short device names may be used when those
devices appear in certain well known locations under /dev/.
This change adds /dev/mapper/ to that list.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Improve 'zpool import' EBUSY error message

When a device is already open O_EXCL by another process the
`zpool import` will correctly fail.  However, the default failure
message isn't very helpful.  It may in fact be harmful if you
take its advise and destroy your pool.

    cannot import 'tank': pool is busy
            Destroy and re-create the pool from
            a backup source.

Improve the error message in the EBUSY case to simply print a
message indicating that the devices are current in use.  The user
will need to manually identify which process has the device open
exclusively and why.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>