call_usermodehelper() should wait for process

As of Linux 3.4 the UMH_WAIT_* constants were renumbered.  In
particular, the meaning of "1" changed from UMH_WAIT_PROC (wait for
process to complete), to UMH_WAIT_EXEC (wait for the exec, but not the
process).  A number of call sites used the number 1 instead of the
constant name, so the behavior was not as expected on kernels with this
change.

One visible consequence of this change was that processes accessing
automounted snapshots received an ELOOP error because they failed to
wait for zfs.mount to complete.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #816
Report realpath() canonicalization error

Rather than just reporting the failure include the passed
mount point and error number.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1153
Add /sbin/fsck.zfs helper

A fsck helper to accomidate distributions that expect to be able
to execute a fsck on all filesystem types.  Currently this script
does nothing but it could be extended to act as a compatibility
wrapper for 'zpool scrub'.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #964