Improve HAVE_EVICT_INODE check

The hardened gentoo kernel defines all of the super block
operation callbacks as const.  This prevents the autoconf test
from assigning the callback and results in a false negative.
By moving the assignment in to the declaration we can avoid
this issue and get a correct result for this patched kernel.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #296