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
MFC after: 3 days
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D34350