Increase default zfs_multihost_fail_intervals and import_intervals

By default, when multihost is enabled for a pool, the pool is
suspended if (zfs_multihost_fail_intervals*zfs_multihost_interval) ms
pass without a successful MMP write.  This is the recommended
configuration.

The default value for zfs_multihost_fail_intervals has been 5, and the
default value for zfs_multihost_interval has been 1000, so pool
suspension occurred at 5 seconds.

There have been multiple cases where a single misbehaving device in a
pool triggered a SCSI reset, and all I/O paused for 5-6 seconds.  This
in turn caused MMP to suspend the pool.

In the cases observed, the rest of the devices were healthy and the
pool was otherwise correctly performing I/O.  The reset was handled
correctly by ZFS, and by suspending the pool MMP made replacing the
device more difficult as well as forcing the host to be rebooted.

Increase the default value of zfs_multihost_fail_intervals to 10, so
that MMP tolerates up to 10 seconds of failed MMP writes before
suspending the pool.

Increase the default value of zfs_multihost_import_intervals to 20, to
maintain the 2:1 safety factor.  This results in a force import taking
approximately 20 seconds when MMP is enabled, with default values.

Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Andreas Dilger <[email protected]>
Reviewed-by: George Melikov <[email protected]>
Signed-off-by: Olaf Faaland <[email protected]>
Closes #7709 
Closes #8495 

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