linuxkpi: move io_mapping_create_wc to .c

Move io_mapping_create_wc to .c because it encodes the size of struct
io_mapping so we move this from the client module to the linuxkpi
module.

Approved by: re@ (gjb)
Sponsored by: Netflix
Reviewed by: hselasky, emaste
Differential Revision: https://reviews.freebsd.org/D34776

(cherry picked from commit 132b00f90613d8cc797137d4c3d2dcb99ba3c690)
(cherry picked from commit 4e94b3542dd6a5c3f60431e811013f4dbf603227)