3246 ZFS I/O deadman thread

Reviewed by: Matt Ahrens <matthew.ahrens@delphix.com>
Reviewed by: Eric Schrock <eric.schrock@delphix.com>
Reviewed by: Christopher Siden <chris.siden@delphix.com>
Approved by: Garrett D'Amore <garrett@damore.org>

NOTES: This patch has been reworked from the original in the
following ways to accomidate Linux ZFS implementation

*) Usage of the cyclic interface was replaced by the delayed taskq
   interface.  This avoids the need to implement new compatibility
   code and allows us to rely on the existing taskq implementation.

*) An extern for zfs_txg_synctime_ms was added to sys/dsl_pool.h
   because declaring externs in source files as was done in the
   original patch is just plain wrong.

*) Instead of panicing the system when the deadman triggers a
   zevent describing the blocked vdev and the first pending I/O
   is posted.  If the panic behavior is desired Linux provides
   other generic methods to panic the system when threads are
   observed to hang.

*) For reference, to delay zios by 30 seconds for testing you can
   use zinject as follows: 'zinject -d <vdev> -D30 <pool>'

References:
  illumos/illumos-gate@283b84606b6fc326692c03273de1774e8c122f9a
  https://www.illumos.org/issues/3246

Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1396
Fix zinject list handlers

The zfs_fd must be opened before calling print_all_handlers() or
the ioctl() cannot be used to the zfs control device.  This brings
the zinject code back in sync with the Illumos implementation.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Silence 'old_umask' uninit variable warning

Recent changes have caused older versions of gcc to mistakenly
flag 'old_umask' in vn_open() as an unitialized variable.  To
silence the warning initialize it.

  kernel.c: In function 'vn_open':
  kernel.c:525:6: error: 'old_umask' may be used uninitialized
  in this function

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ensure --with-spl-timeout waits for spl_config.h and symvers

The previous code was only waiting for the symver file. But the
postinst target of the DKMS script for SPL will not only create
the symvers file, but also the header spl_config.h.

If we are waiting in the configure script of ZFS for the SPL
symvers file, then we also need to wait for spl_config.h.
Otherwise the configure script will abort because the spl_config.h
is not yet available.

On top of that, the function ZFS_AC_SPL_MODULE_SYMVERS is moved
to the end of the function ZFS_AC_SPL to allow both checks share
the with-spl-timeout parameter.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1431