Fix minor autoconf error message inconsistencies

A few of the autoconf error messages were inconsistent with the rest of
the build system. To be specific, the inconsistencies addressed by this
commit are the following:

 * The second line of the error message for the CONFIG_PREEMPT check
   was missing it's third asterisk.

 * A few of the error messages were prefixed by two tabs, whereas the
   majority of error messages are only prefixed by a single tab.

Signed-off-by: Prakash Surya <surya1@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>