Remove spl_invalidate_inodes()

This functionality is no longer required by ZFS, see commit
zfsonlinux/zfs@7b3e34ba5a7ee8d0fda44d214f6f11eb16cdb26f.
Since there are no other consumers, and because it adds
additional autoconf complexity which must be maintained
the spl_invalidate_inodes() function has been removed.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue zfsonlinux/zfs#795