LinuxKPI: change DECLARE_FLEX_ARRAY()

DECLARE_FLEX_ARRAY can be used inside a structure. On FreeBSD due to
-Wgnu-variable-sized-type-not-at-end this yields an error. Use [0]
instead of [] to overcome this.

Sponsored by: The FreeBSD Foundation
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D34350

(cherry picked from commit fab601c5b76a1183c1849222667481fdde948ef9)