Fix dbufstats_001_pos

Implement a new helper within_tolerance to test if a value
is within range of a target.

Because the dbufstats and dbufs kstat file are being read
at slightly different times, it is possible for stats to be
slightly off. Use within_tolerance to determine if the value
is "close enough" to the target.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Closes #7239
Closes #7266