ixl: gcc build errors

Fix minor -Werror issues when building with gcc from -Wredundant-decls,
-Wunused, -Wbool-operations. Also ensure the M_IXL malloc type is only
defined once.

Reviewed by: efj
Approved by: markj (mentor)
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D11414